This is the second time (to my knowledge, may have been going on for a while) that an Xcode beta has been made available to only one of the two (Mac and iOS) developer programs. The Xcode 4.3 beta was only available to iOS developers -- despite containing bug fixes and improvements useful to developers on both platforms. Now the Xcode 4.4 beta is only available to Mac developers.
I really don't see the logic in withholding from paying members of either platform.
You can't use the beta Xcode / SDKs to build apps for the relevant App Store anyway, so while it contains neat new features it's technically only useful for developers developing that particular pre-release OS (whether iOS 5.1 or OS X 10.8).
Interestingly now Xcode 4.3 is out of beta the iOS 5.1 SDK is still only available with Xcode 4.3 DP3 (although realistically there's not much public new stuff in iOS 5.1 since presumably it's all under wraps for the iPad 3).
Xcode 4.2 was also only available to iOS developers.
Xcode 4.1 was part of the Lion pre-release, so I don't think that iOS developers got it during beta, though apparently it was made available for SL users a few weeks after Lion shipped.
It at least makes sense to limit the betas to the Mac program only if the tool in question only works on a prerelease OS (I assume that's the case with 4.4 and ML). But limiting it to iOS developers only makes no technical sense at all.
Edit: 4.4 works on 10.7.3+, so yeah, this is just the usual bullshit.
I really don't see the logic in withholding from paying members of either platform.