Quite possible that this is racism but then again three young men sitting down in a late night eatery may well also be the problem. I strongly suspect the restaurant has had issues with those doing a runner, it may be along race lines it may just be along the lines of a group of blokes . Only the restaurant will know the real truth.
I used to drink a back street bar in Bruxelles. They never asked for a tab even when we supped 40 beers between the tow of us one lumchtime, but a group of french speakers came in and they were asked to pay for each round. I have no doubt it was becasue they knew us but it would be very difficult to prove as if the others had complained the bar did not even know our names to put forward to back their claim.