I wanted to know how I can hide App Store purchases on my iPhone and iPad since they're not hidden on the two devices even though I hid them on iTunes (Mac).
I believe the option to do that on an iDevice has been removed. I've been trying to figure it out too and it seems it's been gone since iOS 6.
By saying hide it, do you mean hide an icon? Or hide it's complete contents (e.g. data, location, etc.)?
If you mean how to hide an icon, check out this video. It's very simple. All you have to do to open the app and search for it in spotlight.
Neither. The OP is referring to the option we used to have to hide downloads from the App Store so they don't appear on your "purchased" section.