The park may have instituted "resort Fees" for a number of reasons. It may just be the way all the parks in the area set their rates and the park would be at a disadvantage if they didn't follow suit.
Example: Park A is $80.00 with a $30 resort fee, Park B is $90.00 inclusive of all fees. Searching for the best deal may very well pull up Park A with it's $80.00 base rate putting Park B, which is actually cheaper when everything is considered, behind it.
It may also be the park is trying to save you and themselves money. If they charged an all inclusive rate, the entire amount might be subject to lodging taxes. If the park is a franchise park, they likely pay royalties on the site rental and not on resort fees. Between the two, there might be a 20+% savings on taxes and fees on that resort fee amount.
In the end, for me it is the total amount I have to pay. I don't care how they arrive at it as long as they don't spring extras on me once I arrive.