People are perfectly capable of choosing whether or not they want to study nuclear energy. They can't choose whether or not they want to be a mage, yet they can still be possessed and turn into horrible abominations.
There are similarities, of course, but there's also a world of differences.
But people normally have to agreed to be "possesed". And if I remember well about the lore of DA, demons and spirits are the exact same kind of creatures, humans make a distinction between both, but they are the same, which means that an "abomination" may be free to control his life and acts, examples: Anders for spirits and Uldred or the bad guy of the Dawn of Seeker movie for demons.
And why are we making a judge about every "abomination" being bad? Most mages turn into an abomination in the middle of a fight to death for his life against people who wants to kill them for the "crime" of studiying "forbidden" (by X persons who thinks they can tell what is right and what not) knowledge, normally templars, which means they do it in a state of complete despair. Other times possesed people bears an incontrollable fear of demons, spirits and the Veil which may affect the outcome of the possesion.
There was a templar on the Circle Tower in DA: Origins who was possesed by a Desire Demon who simply wanted to feel human emotions through him, so she makes a heavenly ilusion on his mind about him being happily living with his family. I let the templar and the demon go. I am curious about how a mage without any kind of prejudge against demons, spirits, the Veil and "forbidden" knowledge would be if possesed by a demon in a peacefull manner (agreeing with it in exchange for power or any other thing), I don´t think said mage would become a jackass of destruction but something like Anders instead, where he can control his life and have the demon inside him, feeling the same feelings he has and being able to voice his opinions too.