They’re “closing the barn door after the horse has bolted”.
They didn’t act immediately; it’s possible they thought it was a skit themselves; and also silly and very human things too happen at these events every time.
But they got a negative backlash, saw the wind was blowing favor against Will Smith, so they did the ban hammer – kind of.
a) He resigned instead of being expelled
b) He was only forbidden from ATTENDING the Oscars
As it’s a by-invitation-only group, they can choose to change their 10 year term or invite him back with cavaets if the winds blow back in his favor.
They might also be playing “the bad guys” by OVERDOING the ban, in order for sympathy to come to Will Smith, and they can then play the reconciliation game later on.
The 10 year period ensures the Oscars get press every year: “Will they let Will Smith come back this year?” etc. And Will Smith’s career will be fine. He’s an excellent actor and he’s not blackballed; he’s doing his mea culpa properly.