To prevent a tragedy of the commons type situation. I may think that local businesses are better than online monopolies. However, my purchasing decisions, whether I change to local or buy online, will have a negligible effect on the market. Might as well save myself some money if I won't be saving local businesses.

If everyone thinks like this then the tragedy of the commons occurs. Everyone prefers local businesses to exist over the online monopoly, but nobody changes their purchasing behavior from what is cheap and convenient. By voting restrictions on online monopolies people avert the tragedy of the commons by coordinating (and compelling) what they want.