Win 8.1 is a point upgrade. Some new features, mostly bug fixes. .x updates have always been free AFAIK
Mavericks is a full upgrade, while not as radical of a change between 10.8 and 10.9 like Windows 7 and Windows 8, its still a major upgrade and to that point I don't think we'll it for free (and for the other reasons I already posted)
To be honest I think 8.1 is a bigger change for Windows over 8 than Mountain Lion was over Lion.
The last point release for Windows was 3.11 back in 1993. It was charged.
(Clarification, 3.11 wasn't charged over 3.1. To upgrade from 3.0. to 3.1 or 3.11 was charged)