Exactly the same limitation goes for all iPad models.
Or most Android tablets for that matter. Advantage of Android is that there are several variations (aka custom ROMs) available, but usually not when these tablets are introduced.
There are Android ROM's available for a few older Windows Mobile 6.x phones, so let's wait and see
Well, Apple is Apple. They more or less started the tablet madness and were smart/moral-less enough to close the new platform. I'll simply ignore them (never had any iPhone nor iPad).
Although I do not own Android tablet, there seem to be a lot of various guides on how to install apps not in the Google store on your Android phone or tablet. These ways may be much less comfortable than using the store, but they are clean, legal ways and no hacks. So, I do not consider Android closed.
Now comes WinRT. On tablets with Windows RT totally closed, on x86 desktops with normal Windows 8, partially closed (you can run legacy any apps, but only Ms approved new apps). That is a new thing - closing desktops. That irritates me a lot. Mainly because the WinRT API looks pretty good compared to iPad's or Android's API. I would love to use it to make Metro app, but under the current conditions, I refuse.