Given the implication of your question that there could be a technical
reason to avoid the reason is because, which bars answers simply
referring to the fact that the construction is part of actual current
The reason is because is WRONG; not redundant but illogical.
The syntax of the conjunction because is:
A because B
where A and B are complete statements. But the only way to make ‘The
reason is’ a complete statement is to read ‘is’ existentially. So,
The reason is because it rained yesterday.
tries to mean, all it succeeds in meaning is
The reason exists because it rained yesterday.
which is clearly not the same as
The reason is that it rained yesterday.
For those reluctant to accept my syntactical statement, it may help to
consider that the etymology of because is by cause.
A because B
A by the cause that B
which is to say
A for the reason that B
The reason A is because B
The reason A is for the reason that B
which can be filled out into
The reason he doesn't eat is for the reason that he is not hungry
But this clearly means that the reason he doesn't eat exists for the
reason that he is not hungry, which may be true, but is not what one is
trying to express.
It is still possible to reject this argument from etymology. But, as
far as I can see, the only reason to ascribe a different syntax to
because than that I have given would be to accomodate the construction
in question; and, if this kind of trickery is allowed, anything can be
defended and there is no technical reason to avoid anything.