Because your intentionally tampering with food which is felony. You get a 15 second laugh, and then the person you pranked can turn around and get you sent a prison for up to 15 years and $10,000 in fines.

Sure, they deserved it, but it ain't worth going to prison over.

Edit: Okay, am I getting downvoted because I'm wrong, or because you guys just don't like the answer?