Though it could be argued that the ease with which their games can be modded obviates that inclination to a certain extent (although that is also a fairly major selling point).
You're right, but you might reconsider how you feel about this when you realise that they're trying to
monetise mods.
Personally, I don't mind if mods are monetised as long as it can be done without damaging the modding community, and it's done in a consumer-friendly way. Some mods are worth paying for. (Bear in mind that modders would obviously have the option to keep their mods free.)
But anyway, yes, Bethesda and Valve want to turn mods into paid DLC.