I checked the grammar: "promotional" is the description of tool, thus "promotional" tool is right while the first is wrong.
Is it true? Why are there so many cases that those two are exchangeable?
If you said the words "promotion tool" to a native speaker of English, that person would probably think you were talking about some kind of tool used by Human Resources to assist in bumping some one up one or more job levels. That is because promotion is a noun.
Promotional, on the other hand, is an adjective meaning "of or relating to the publicizing of a product, organization, or venture so as to increasesales or public awareness."
Now, it is not exactly incorrect to say "promotion tool" to mean "promotional tool" — English uses nouns as adjectives all the time. It's just that in this case "promotional tool" is a very common term for what it describes, and most people use it. Promotion tool is seldom heard (I never heard it used till now) and sounds awkward and would certainly be confusing, as I mention above.
The NOAD gives this definition of promotion (among others):
It also defines promotional as
I would understand promotion tool as referring to a tool used in the business of organizing publicity, and promotional tool as a tool related to publicizing a product.