WS: well, Tyrion was in a different position. He was embarrassing but hey, he wasn't the heir. Until Tyrion was about seven Jaime was clearly the heir, so no problem.
Even afterwards, Tywin tried to carry on as if Jaime was still his heir, apparently, and simply refused to acknowledge Tyrion would inherit. Nobody seems to have bothered to correct him, largely due to the fact that Tywin was a hell of a lot more powerful than Randyll was.
Ultimately Mace Tyrell would have had to judge who Randyll's heir was if he'd tried to disinherit Sam, and he would legally have had no other choice than to confirm Sam. Whereas only the king could have overruled Tywin if he had simply refused to make Tyrion his heir, and that would have risked rebellion or war.
For a clever man, Tywin was very good at denial, you notice that?