I'm confused... If the 'Servants' are hostile towards outsiders, how do we know they are slaves and not serving the Falmer of their own free will? I mean, if they were slaves, wouldn't they want you to free them? Why would they defend the Falmer if they are there against their will? Does the player get to talk to one?

They have nothing interesting to say.
I always thought that the Falmer got them addicted to the same mushrooms the Dwemer used on them.
That way the servants would not really try to run away or betray their masters as that would lose them the source of these mushrooms.

Well if they were slaves for years, the falmer could have easily broken their will and spirit. So they have no desire to escape because they pretty much think theres no chance of escape or that the falmer cant be stopped.

That Amy smart girl in real life comes to mind for example. She was abducted as a kid and missing for a year. When they found her as a slave for some packie dude, and the cops arrested him, it took them forever just to get her to admit who she was. She was that afraid, and that was only after a year of imprisonment. This guy was going to public places with her where she could have easily been like "hey! help me!" for over a year, and she didnt even attempt to escape.
Just goes to show the power of fear over someone.