What we've learned from our history is the stronger power typically enslaves the weaker, why would you think non-terrestrial intelligence wouldn't enslave us?
Historically there has been an economic advantage to enslaving people; if you enslaved someone you could get them to do work for you, so you didn't have to do the work yourself.
A non-terrestrial intelligence, contrariwise, would either not be present on Earth (in which case it wouldn't have the ability to enslave anyone on Earth), or if it did get to Earth, it did so by harnessing enough energy to make the trip across interstellar space. Any species capable of harnessing that much energy on its own is unlikely to need to enslave anyone to get its work done. It would be like you or I 'enslaving' a hamster to generate electrical power for our house -- there's not enough benefit to make it worth the effort of doing.