Not to be a random fee defender here, but I could potentially see a reason for charging the fee on this tickets.
specifically, With the print-at-home tickets, you cannot simply use the existing ticket scanners at the turnstiles that you use with every other ticket media. This means there is a small additional cost in supporting these tickets in the equiptment and time to scan them as you enter the park.
i could see a possible valid reason for passing these unique costs along to the one type of media that incurs them. (Now.... is that extra cost really $2.15 per ticket..... dunno).