If a seagull is stealing chips from someone, odds are there are plenty of other seagulls around to witness their compatriot getting merked.

Seagulls understand that stealing from humans is risky - that's why they generally do it very quickly. The ones who fail suffer consequences for their failure, same as stealing food from any other creature. It's the risk/reward calculation any scavenger has to make.

Sometimes they calculate incorrectly. They get forcibly removed from the gene pool.

Of course, it's also illegal in a lot of countries to harm seagulls, so in that sense, he was in the wrong anyways.