Check the license. Different open source license have different requirements about what kind of changes must be contributed back to the origin.
While there are a lot of cases where an author picks a license out of ignorance, most of the time, the license that author picks reflects the author's ethical position.
Most open source license are written in a manner that is technology-agnostic, forking to a different code host or DVCS would not be considered distinct than forking to the same code host and DVCS. It is a feature of many open source license which allows forkers to use whatever technology they see fit for their own purpose as long as the requirements of the license is fulfilled.
Forking to a different technology does add a layer of difficulty for contributing back, so that should be part of your consideration whether it's worth it to fork to another technology. These technical difficulties are now lessened since there are now very stable and high quality bridging between Mercurial and Git.