.net has never been part of service packs so I doubt it .... its stupid but its why they do.
I don't think it's stupid since:
1) .NET setup and installs waste a lot of space
2) .NET is released more often than service packs (in the time between SP2-SP3 there will have been 3 .NET releases (2.0, 3.0, 3.5)) so in the future people who will install XP SP3 will get rid of the then old .NET framework in favor of a more recent version
3) .NET isn't required by XP itself (unless we're talking about windows media center that however is still stuck at 1.1 with the latest cumulative update)
4) not anybody needs .NET just like not anybody needs java. I actually have only 1 application that rely on .NET (nLite) between the dozens I've installed so it's not that all essential
probably it would be better for MS if they bundled Silverlight and also added an option to install Windows Live Suite at the end of the SP3 setup.