I think, as said in a previous post, that den claimers are usually under 10, so they probably don't know how to make their own maps... Or are not skilled enough...
As for what I do when I meet one, well, it depends on the situation. If I'm with a pack, I'll tell them that they can't claim territory and that it's against the rules. But if I'm alone, I'll just move on. I mean, seriously, Fluorite Plains is big enough for everyone. If they want to RP, or whatever, in a particular spot, and not be interrupted; let them. Why not? As Iv'e said: Fluorite is a big enough map there there will always be free space in it. And as for other maps, well, Iv'e never actually seen any territory claimers anywhere besides Fluorite... Maybe that's because all the other maps are too small for territory claiming...