Do you really find that a surprise? If you look at Mephisto's wording, he clearly says he will punish any mortal he feels is mistreating his temple. This does not mean it is considered a sanctuary, but a place to be respected by others. I would think that perhaps summoning from a temple would be OK as it would not sully the temple grounds. Fighting however, would mess the floor and walls up a bit. Of course, some gods may like that sort of decor.
Karamis, of the grass-stained feet