Re: Google itself is the reason for sideloading
"I wonder how many people sideload an app, and then not reset the device to no sideloading?"
Marshmallow allows sideloading to be enabled on a per-apk basis (ie, "allow installation of this file only")
If you don't have some kind of scanner checking files (even off the google store) before installation then you're taking a risk. Malware has pigyybacked on all app stores at times, even on Slurp Play.