No, netflix isn't side loading apps. They are asking you sign up for the service on their site, not through the app to avoid having to pay Apple the 30%. A user still must get the app through the app store. There is no way to get an app on the phone that is not signed by apple..
You can now get developer certificates/provisioning profiles without paying the $100 you used to, just by having an Apple ID. You can replace the signature of an app given to you in binary form or build one given to you in source form via Xcode, which is also free.
Sorry I misspoke. I should have said only that iOS allows side loading. I thought Netflix was utilizing this.