First off, when you comment that the fee is optional, its optional in the sense that they could raise their rental prices and not have the fee--the car rental company has to pay to use the space at the airport. Because of all of these "extras," when I compare car rental prices, I always base it on the total price and not just the daily rate quoted. Sometimes these "extras" can add up to be more than the base rental price!

As far as getting these fees waived/discounted, I find that a lot of this is at the discretion of the agent, and I think at least part of it is just dumb luck. Looking at recent rental receipts from BOS, one time they did charge me the airport concession tax, the other time they didn't--but both times they did hit me up for the convention center charge (a lovely $10.30 per rental).

At SFO, they waived the "transportation fee" once several years ago as a courtesy because of a mixup over my reservation, but yet another time when I took the SamTrans bus to the car rental facility, avoiding the airport shuttle altogether, I enquired about having this fee taken off, but they said they couldn't do it.

So, I think the moral of the story is twofold: it doesn't hurt to ask, but don't get offended if they don't take it off; and always consider the cost of a rental on the total cost after fees and taxes, and not on the daily rate quoted.