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I often hear people say things like “That's the reason why I....”

As far as I know, “That's the reason I...” would still be grammatically correct, but I can't find anything stating one way or the other.

Can someone please tell me whether I'm right or wrong?

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Yes, that perception is correct.

The reason (why) that perception is correct is that why is a rather special relative pronoun.
Indeed, it's a pronoun that can only refer to one word: reason.
Try it with anything else and you get garbage:

  • the reason why he did it
  • *the cause why he did it
  • *the intention why he did it
  • *the effect why he did it
  • *the thing why he did it
    (ungrammatical phrases and sentences are marked with an asterisk)

and these are nouns that could make sense with a Why relative pronoun;
try it, if you dare, with nouns that couldn't, like rock, salamander, or durability.

Relative why can be freely substituted with that, like any restrictive relative marker.
I.e, substituting that for why in the sentences above produces exactly the same pattern of grammaticality and ungrammaticality:

  • the reason that he did it
  • *the cause that he did it
  • *the intention that he did it
  • *the effect that he did it
  • *the thing that he did it

More important, why refers to an adverbial clause or phrase of some sort in the relative clause -- certainly not a noun phrase -- and therefore it can't possibly be the subject of the clause. This is important because relative pronouns that are the subject of their relative clause (like the man who/that came to dinner) cannot be deleted. But adverbial wh-words -- like why, where, when, and sometimes how -- can't ever fall into that category.

This means why -- or that -- can be freely deleted after reason. I.e, deleting why in the sentences above also produces exactly the same pattern of grammaticality and ungrammaticality.

It's not a matter of redundancy; all pronouns are redundant, after all.
It's just that why is very limited in its distribution.

Not quite as limited as how, however.
How can't be used at all as a relative pronoun;
one may use that, or nothing at all, but
how (which refers to way) is ungrammatical as a relative marker.

  • the way that he did it
  • the way he did it
  • *the way how he did it
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    Good explanation of why it's optional in this case, although I'm not convinced that reason is the only reasonable antecedent of why. For example, the explanation why is a common usage, and I don't think you can freely substitute that in that case either. (Perhaps it's already a contraction of the explanation of why?)
    – B. Szonye
    Commented Jul 20, 2013 at 20:06
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    I would insert for in the explanation (for) why S.
    – John Lawler
    Commented Jul 20, 2013 at 20:14
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    @BraddSzonye: In "an explanation why [...]", the why-clause is an interrogative content clause, just as in "to wonder/explain/ask/examine/know/etc. how/why/whether/etc. [...]"; that's definitely distinct from the relative clause found in "the reason why [...]".
    – ruakh
    Commented Jul 20, 2013 at 22:36
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    An NP Complement embedded question... Yes, that works, for derived picture nouns like explanation, provided explain can take an embedded question complement; and it can: He explained [why they had to fingerprint me].
    – John Lawler
    Commented Jul 20, 2013 at 23:44
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    Note that neither "why" nor "that" are pronouns here.
    – siride
    Commented Jul 21, 2013 at 3:55
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Why and the reason are redundant when used together. From Merriam-Webster Online:

why : for what cause, reason, or purpose <why did you do it?>

Therefore, these all mean the same thing:

That's why I. . . .
That's the reason I. . . .
That's the reason why I. . . .

While many people prefer to avoid redundancy in writing (“Omit needless words”), a little redundancy is often helpful in speech to improve flow and to compensate for the vagaries of hearing and attention span.

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    +1 for the redundancy bit. I dislike redundancy as a programmer as try to avoid it all the time, but your explanation allowed me to better realize why redundancy can be good in the real world.
    – user48179
    Commented Jul 20, 2013 at 19:42
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    Thanks. By the way, it's a good idea to wait about a day before accepting an answer, so as not to discourage other people who might have even better answers. (And John Lawler's answer is arguably better than mine.)
    – B. Szonye
    Commented Jul 20, 2013 at 19:55
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    I see. I'll be sure to keep that in mind for future questions. I like your answer simply because it's very "to-the-point", but his is more accurate and informational. While I like both answers, his is probably a better fit now that I think about it. I'll unaccept this and see if anybody else has any additional input before choosing a correct answer.
    – user48179
    Commented Jul 20, 2013 at 19:59