In most places, restaurants aren't allowed to give them free food. (edit: as a commenter pointed out, may be more of a police policy than a legal one) Yet you'll always see restaurants frequented by cops on-duty - guess which ones are breaking those rules.

It's a enormous stretch to call it bribery. Amazon isn't asking the police to look the other way. They aren't encouraging anything unethical or a breach of trust. Any more than a free laundry soap sampler in the mail is bribery.

Should the police have regulations about this sort of gift? Should, they probably do, and those that don't should.