Interplay decides where Fallout 1, 2 and Tactics are sold and takes the revenue from them, as it always has. Bethesda buying the IP didn't preclude the former, and I doubt any subsequent purchases will affect it.
One of the effects of the Fallout MMO lawsuit was that Interplay only has the rights to sell FO1, FO2, and Tactics until December 31, 2013. After that point, it's up to Bethesda.
However, the recent Zenimax business model of "all Steam, all the time" makes me think it's unlikely they'll keep the games here. Their decision to pull the Doom games from sale entirely (they're no longer available ANYWHERE, unless you cough up $100 for the id Collection on Steam) after the BFG edition of Doom 3 launched really worries me as well.
I'd definitely make sure to buy copies of them before then, just in case.