As a restaurant owner who offers wifi to it's customers in DC, I've already assumed the risk of having you take a table during my busy time by offering wifi. Now, maybe some businesses don't think this point through, but I wouldn't be concerned about taking a table for an extended period of time. As an owner, we're hoping you either:

A. Frequent our establishment more often because we have wi-fi.

B. Make an impulse purchase while you're there that would otherwise not have been made.