Originally Posted by docbert

This has been the case for some time on most G fares. The reason you likely haven't seen it much is that G fares aren't all that common - as stated they are normally used when they are competing with a fare sale from one of the other airlines, especially someone like Southwest.

Given how cheap these fare usually are (eg, there's a $29 inc tax SFO-LAS available at the moment!), I don't have an issue with them limiting combinability like this, especially given the easy workaround due to the fares not being only roundtrip like they could be...