True randomness is anti-resistance since we can't analyze your choices.
I agree in theory. But practically, we don't know ftl's alignment. If he's a Spy, allowing him to chose would be fairly bad for Resistance, and here's why:
If Spy ftl chooses to give Open Up to a fellow Spy, that person must then reveal to player X. If player X is Resistance, they declare the recipient of OU to be a Spy. Since ftl chose the recipient, I do not want him in the pool to be player X. This means of 5 players, 4 are Resistance and will expose his teammate. Very bad odds for him.
Thus, Spy ftl would choose to give OU to a Resistance. Again, eliminating ftl from the Player X pool, there are 2 Spy's amongst 5 players who could see the reveal. 40% of the time, Spy's will finger an innocent Resistance, causing massive difficulty for the Resistance.
I don't know about you, but 40% is worryingly high for my tastes.
As Resistance, ftl choosing makes the odds even worse, as you've taken out a Resistance from the pool, and added in a Spy.
Thus by randomizing, ftl's name can be added to the "player X" pool. If he's Resistance that's great. If he's a Spy, we've forced him to possibly hand OU to a partner, in exchange for ftl being in the pool. Resistance still gets 4/6 odds of being chosen at that point to be player X.