I think if it were 'optional' or a setting that pool creator could modify, then we run into a problem with "owner is MIA" or potential hijacking scenario where someone makes a pool between 2 popular tokens and messes with the settings into something not ideal for anyone. It would also create a contrast that made any pool with fees look like some type of negative thing you're supposed to avoid through circumventing around between the pools that turned it off.. and whatever charity it offers to the people swapping is coming from the opportunity cost of the liquidity providers, in a way. For those reason I think the universal fee is better, even though I personally love to have more options available.