The curious thing about the word "slave" is that it originates from "slavs" i.e. people living in slavic countries, who were forced to slavery, yet we aren't freaking about that (I'm a slav by myself), it's just a word.
Calling it "sideloading" instead of "installing" software successfully cements the notion that it is somehow not a completely normal thing to do. That's problem solved for the Googles and Apples of the world.
See the history of "jaywalking".
See the history of words such as "jaywalking" or "carbon footprint" and how their usage cements the respective ideas.
There is no choice of words that will make it normal to install mobile apps from anywhere other than an app store. Whatever word we use will take on the meaning of doing something unusual.
"Sideloading" doesn't have an inherent or deeply ingrained negative connotation. I don't see a reason to try to change it.
Let me just "sideload" an app onto my laptop...
Does that make sense at all? "Sideload" and not "install"?
It's the way our brains work - the intention doesn't necessarily matter. Next time you're pissed off, try expressing out loud how "darn peeved" you are and watch how much words change how we think and feel