This is due to the fact that Odamex uses W_IsIWAD() to determine if the file is okay to download which will return true for all IWAD's, not just commercially available ones.

A robust solution would be to make that check more nuanced.  Instead of returning a simple true and false, it could perhaps return a pointer to the appropriate doomwadnames entry on success, or a null pointer on failure.  The doomwadnames array could then be expanded to have more robust information, such as if the IWAD should be downloadable.

I seem to also have some memory of the canonical Freedoom WAD names being the same as the commercial WAD names.  If that's the case, we should probably also handle that case properly.

Freedoom does name itself doom2.wad, and I assume ultimate freedoom will also name itself doom.wad.

Probably the better way would be to check the wad against a checksum database of known commercial WADs (to check for different release versions of the WADs).