They do not call it 7.0. They call it 7 because it is the 7th version of the OS (as they count it). The kernel does get the number 6.1 because of compatibility concerns. A lot of apps look for the main version number (6) before installation. Becasue Windows 7 does use the same driver models as Vista, this does make a lot of sense.
If you really believe that the world's most ubiquitous platform can't handle a version number increment without failure, then sure, Windows 7 needed to be numbered as 6.1 internally while beta testers are told it is version 7000.0 and it is marketed as version 7 of the NT kernel OS (a system which never had a version prior to 3.1, but whatev).
Regarding the "Snow Leopard is Leopard SP1 !!!" comment, Apple has already released six free "service pack" updates to Leopard, and will likely pop out another one or two before SL appears. So no.
Additionally, Snow Leopard has a number of significant kernel updates that warrant a new major version number update (as Vista did), as the next segment will indicate. And of course, it wasn't the 6.0 numbering scheme for Vista that cause compatibility problems, it was changes to the driver model and other upheavals that did. So Microsoft is lying about having its hands tied at 6.x.