I'll have to agree with raynor - I think they're maintaining the partnerships purely because you're too far ahead for them to want to fight. I base this on the fact that I had an ally declare war on me within 5 turns of signing a Partnership treaty (and watching the empire's attitude change from Brotherly to Murderous). I think he got upset about all the worlds I'd colonized that he wouldn't get to colonize...
Anyway, in the same game, I've maintained a treaty with one neutral empire, even though they're "Murderous" towards me (again, I think they're upset because I colonized a couple worlds in their system
).