— it's a matter of perspective I guess. Technically, NW exports run "on linux". They work perfectly well on Redhat, Centos, Scientific Linux, basically where all the corporate users sit.
Using it for games, where people use Ubuntu, Mint, Fedora, Mandriva etc. probably isn't the main focus of NW in the first place, that's why they have gotten away with it so far. But their forums are quite full of angry rants, for that matter.
As to Scirra.. the main argument pro NW was probably the ease of use and good performance - on windows.
Without accusing anyone of anything, I'd go as far as saying the linux exports probably were not tested too extensively when deciding to go for NW as a wrapper.