I suspect it may be illegal. At one point years ago the local McDonalds here had a promotion where if on election day you brought in your voting stub showing you voted, they would give you a free Big Mac. That got shut down by the Elections Office as some sort of voting violation and it didn’t happen again.
Check this out:
https://www.law.cornell.edu/uscode/text/18/597
If they gave me a babe, MAYBE.
But a Big Mac??
Dnc in houston is hosting a free concert at Toyota center in houston to get people to come out and vote... isnt that nice?