The word you are looking for is "preventive".
No, it's not. The usage you're complaining about is perfectly valid.
"Preventative" has been in use since 1666 as an alternate pronunciation and spelling for "preventive".
In some regions (including where I grew up - almost in the center of the region natively speaking the "radio accent", which has been the de facto standard speech for the U.S. since the advent of commercial broadcasting) it is the preferred form.
If you want to be a spelling NAZI, you should avoid being provincial about it. Check the online dictionaries before correcting others, to distinguish between being helpful and imposing your local speech on others.
Unlike French ("a dead language spoken by millions"), American English does not have a regulatory body prescribing an official standard (though some educators have tried, since at least Daniel Webster). It grows and changes by usage. Dictionaries play a game of catch up and try to document how it's realy used.
(Yes, I know how it grates on your nerves when someone uses a different spelling or pronunciation than you're used to. I feel the same way when my wife pronounces "legacy" as if she was talking about a ledge. But apparently that's actually the first pronunciation listed in The Oxford.)