Can't help but chuckle a little bit at the irony.
One of the arguments against this is not knowing what your final price is going to be / not paying the advertised price etc.
In reality, pretty much every transaction in USA is like this, as all your prices are ex-tax anyway, so you never pay the advertised price.
As for the argument about overheads involved due to the park collecting and sorting out the printed tickets; I find it a little, well, a little weak as an argument.
I buy most my airline tickets online and print my ticket (here too I have to pay a 'booking' / 'transaction fee' PER PERSON & PER FLIGHT ). When I arrive to check in, I hand my eticket, they check me in, and then they throw my ticket in the bin if I don't want it any more - no-one goes around collecting all the etickets.
As it goes, I'm totally against unavoidable surcharges added onto the price of things whenever I buy anything - it's mostly illegal to work like this in the UK (and most of Europe) but the grey area is what can be considered an 'avoidable charge'
Oh, and for what it's worth, I just bought & printed off etickets for Alton Towers and Thorpe Park - no printing fee charged, and still got online discount (actually, I also get corporate discount due to a deal my company has with them )