To reproduce: boot F18 Beta TC3 DVD, to the hub screen. Note the 'software selection' spoke has 'GNOME desktop environment' pre-selected, and doesn't have the 'warning triangle', i.e., you aren't required to go through it, you can proceed with installation without going into that spoke. Now go into the 'installation source' spoke and change the source from DVD to 'nearest mirror' (on the network), and return to hub. Now the 'software selection' spoke has the warning triangle, and you can't proceed with install without going into it. But why? 'GNOME desktop environment' is still listed as being selected, and anaconda should be perfectly capable of selecting the packages from that group from the remote repository (instead of the DVD) for installation. There's no valid reason I can see to suddenly require user interaction where none was required before. 'low' severity as this is really just an inconvenience, doesn't break the install or anything.
Once you've changed your installation source, though, you've fetched a new set of repo metadata. This could be completely different from what was originally fetched, and the package selections could be anywhere from completely valid to partially valid to completely invalid. We try to apply your selections to the new repo metadata, but who knows how successful we'll be. Once you've made this change, you really do need to go back and double check everything's still as you want it. You're only lucky because you know what you're doing and chose another similar Fedora tree. Other people will not be as lucky.
well I see the theoretical point, but are there really cases where people pick a remote package source that's that far incompatible with the official repos?
I am positive that anything people can do, they will do.