I did boxing before muay thai, when i boxed, we sparred hard but we wore the head gear with the face bar, and because its only punching, the worst thing to happen would be getting the wind knocked out of you, but no injuries. And the facebar head gear offers so much protection, its like wearing a helmet, sparring like that was really fun actually. With Muay Thai, its much easier to get injured so you spar lighter. This may explain the different mindsets, although it doesnt explain the MMA guys attitude towards sparring as most of them go all out.
(yes you can get injured just by punches)