I'm pretty sure that all those US theme parks that charge way more than Towers don't do it to encourage peope to use public transport!! Disney charge a lot to try and get you to use their own private bus from the airport, but that is just so they can keep you on property and stop you from visiting their competitors.
I was in Florida last week & SeaWorld were charging $25 for "Regular" parking. Didn't do Disney or Universal, but if Mr Google is correct, they also seem to charge $25 a day. If you've seen the vast sizes of the Disney parking lots in Orlando you can quickly work out that Disney are making a fortune on car parking - and this is before they relieve you of over $100 per person for walk-up park entry (no BOGOF's here!)
Many places in the UK that can be hard to get to by public transport (e.g. out of town retail parks, supermarkets etc) offer some form of free parking appropriate to the site. Sure there is often a time limit restriction, or pay & display with the fee refundable on making a purchase in the shop, so as to deter abuse of the carpark. Similarly, motorway service areas have to, by law, give 2hrs free parking as this is seen as appropriate time for a break in a journey.
The issue with Alton Towers charging for parking is that the public transport options to the park are very limited. In addition, when the parking charge was introduced it was widely stated it was to help fund the relief road, which was canned over 10yrs ago.
Just to prove that tolls / charges can be abolished - look at the Severn Bridge. OK, so the bridge is funded by the government / taxpayer & Merlin is a public company. Even so, I bet the up-keep on the Severn Bridges costs way more a year than it would cost Towers to resurface all the car parks & internal roads... on an annual basis.
On the subject of the state of the internal roads & carparks atTowers - I am sure that if they spoke to some of the visitors over Easter they may be able to get a cheap deal on a bit of tarmac work...!