XDA's azrienoch explains 5 myths of custom Android OEM skins. (video)
Hopefully, this video clears many myths and mis-conceptions about Android, its license, AOSP, OEM 'skins', ROMs and the Holo Theme.
Android OEM skins aren't 'skins'. They are completely different versions of Android. Choosing between an OEM version of Android and AOSP on start-up would require a dual boot. Android is not open-source everywhere. The kernel is licensed under GPL and the rest of AOSP under the Apache License, but everywhere else, it is proprietary. You can take and use Android for free and have no obligation to 'give back' to the AOSP project. As soon as you use it, it is your property, and you can do what you want with it. This is why ROMs such as MIUI are proprietary while Cyanogenmod is open-source because the CM team chose to be. AOSP is not 'how Android is supposed to be' or 'how Google intended it to be'. Google wants manufacturers to take it and make it proprietary, so they can have their part of the 'cake' and everybody is happy. Google is not unhappy with OEM versions of Android. Google only controls the compatibility standards for displaying and running apps. If you are compatible and approved, you can include Google Play and all of the Google Apps in your proprietary ROM. The 'Holo Theme' is designed for uniformity in app design, and is not required to be used system-wide. As long as you have the Holo Theme in your ROM for apps to call upon, you are compatible.