It can't do that on other sites, it's a technical limitation. Part of the reason why they push AMP and provide every website with free CDN hosting is exactly to enable caching. If the "purpose" of AMP counts as cheating, then sure?
Once signed exchanges become a thing, that may change, but as seen in this thread, there's a lot of push back for that.
An arbitrary technical limitation. You could preload any site you want. Google wants to push AMP, so they only preload those.
Well of course it's cheating if you compare load times between pre-loaded sites vs. not preloaded ones. And then argue that "AMP is faster", which is obviously wrong because the conditions are not the same.
Once signed exchanges become a thing, that may change, but as seen in this thread, there's a lot of push back for that.