I’m sure there are exceptions, but I’ve been to plenty of shops with “No Sitting on Bikes” signs, and every one of them let me sit on the bikes I wanted to. This includes Harley dealers.
The signs are usually there to keep window shoppers from climbing all over the merchandise, scratching paint with their belt buckles, smudging chrome with their fingerprints and smacking taillights with their feet while they learn how to dismount. The trick is to talk to a sales guy, tell him you’re looking to buy a bike and that a friend said you should check out model X. Let him tell you all about it, then say that it looks awesome, but you’re a bit concerned about the ergos. Ask a couple different ways about comfort and before you know it, it’s “well, why don’t you sit on it and see how it feels.” Now you just need to try to do so without scratching paint with your belt buckle, smudging chrome with your fingerprints and smacking a taillight with your right foot.
Even the local Ferrari dealer gives test drives (unless you drive up in an old beater and wearing a Hooters t-shirt). )