Yes, it is about remaining independent. The ability and license to use FOSS software can not be revoked. With non-FOSS software it can happen any time and with it all the hard work that went into it can not be used anymore. Allowing any one entity to withdraw something that would be part of the OSM project or that the OSM project would depend on is a bad idea.

It's not that I hate non-FOSS software, it is more that I would hate to not be able to use it anymore after I put work into learning to use it and into improving it. And as I'm a programmer I would like to be able to improve the software I use if I feel the need.

Also it should not be forbidden to edit OSM maps with non-FOSS software (someone who chooses to do that should be allowed to), but such software should not be advocated either.