Because it'll cost you your job.
Dale Gibson of The Triangle Business Journal reports here that a waiter at The Angus Barn in Raleigh, NC posted on the internet a copy of Peyton Manning's credit card receipt from a recent dinner there. Manning may not have a job, but he tips like a boss! Not like Mr. Pink. (Go to YouTube and search for "Reservoir Dogs - Tipping Scene" -- you'll find a number of hysterical, albeit completely NSFW videos about tipping etiquette).
Back to Peyton, according to the TBJ article, once the owner of The Angus Barn learned about the check, she quickly fired the waiter. "This goes against every policy we have," said the owner. "It's just horrible."
Well, I don't know about every policy, but certainly the social media policy. Not a bad idea to include in that policy a guideline for employees to refrain from sharing information about customers that the customers themselves would not share with the world -- like patient x-rays, for example.
(h/t Jon Hyman)