It's legal as long as the beer has been properly labelled from the manufacturer for the exact volume of beer they've packaged, and the kegs are permanently marked with the manufacturer's name, BUT (and it's a big but) no bar owner on the planet would have corny hookups in his setup when 100% (+/- 0.00000001% of nanobreweries) use sanke kegs.
I've heard of the odd nano serving out of cornies in his tasting room, just because that's what he had when he started homebrewing and decided to just stick with that, but any distributed beer will go into sankes. If you're a brewpub owner and want to serve beer, your hookups are going to be 100% sanke.