You are correct on all counts. It is a ploy to separate you from your miles. Read the fine print on those donation sites - they get the tax write-off, not you. You are donating $0 to charity - you're just voluntarily erasing your miles from the books.

The airlines, like all major corporations, allocate a defined portion of their budget to charity donations, obviously much in the form of free flights. That's fine and great - but it has nothing to do with whether you donate miles or not.

If you really want to donate miles, offer a free ticket to a local charity that has a need to transport people. Logistically, it's no longer as easy as redeeming a paper certificate and handing it over to the local children's hospital or whatever - but if giving away miles to charity is really what you want to do, it can be done. Opinions vary on deductibility, but worst case at least you know your 25,000 miles directly translated into a free trip for the charity.

The catch of course is that a lot of people would love a way to pool orphaned miles to put them to good use. It can't be done, and that's by design.