I can completely see how you thought that the party would be in both parks, by the way that specific sentence (which you pointed out in a post) was worded. The way they phrased it, it made it seem like party guests could access both parks with a special ticket during the party. In fact, my first thought was.... well, how much extra is the special ticket? I thought it would be something separate from the regular MHP tickets.
But you're right -- these specific price increases for only one park during the party seem extreme. Nothing is being added into MHP this year that wasn't there last year, as far as we know. Same party. Same park. So, I guess these tickets are targeted at the people who want a small taste of DCA's Halloween decor, but who are mainly just focused on the actual party. They would have time to run to DCA and see what's there for the season, and then get back to DL in time for the party.
The problem is that the people who had only planned to do the party in DL that day, and were going to do DCA on a different day, will be paying extra money for their MHP ticket to just go to one park.