That's the bit that pi*sses me off. Microsoft is playing stupid politics with the Windows RT devices; there's no technical reason why it has to suck. It just sucks because someone at Microsoft decided it should.
I don't think it's politics, I think it's common sense. Windows RT is purely meant for tablets, and although they make some concession to being sort of laptop-hybrids with Office and the touch/type covers, that role is more properly reserved for Windows 8 tablets which can run desktop apps.
One of the major selling points of iOS for phones and tablets from a layman's point of view is that you don't have to worry about crappy installers, UI that is unsuitable for touch, and malware. If you brought over the ability to get desktop apps on Windows RT with a simple recompile, you bring all that garbage with it, and if the thing gets even remotely popular there will be malware for it (something that Android is already experiencing). Microsoft made the explicit decision to target Windows RT at roughly the same segment as the iPad, with the addition of Office to do "real work" as a nice bonus so you don't always have to carry both a laptop and an iPad. Not to mention it makes the support story and compatibility going forward a whole lot easier.
The reason they can do this is simple: because there is still regular Windows 8 which can do all those things.
The only mistake Microsoft has made here is in not communicating that difference more clearly (hypothetically, at least; I don't think anyone has at this point seen real complaints from people who bought an RT device with the expectation it could run old Windows apps). Having that limitation on Windows RT makes perfect sense to me.