Obama already did his Hishtadlus, by having this french terrorist on a watch list since 2010, banning his entry into the USA, so as far as Obama is concerned, putting him onto this watchlist, was his way of condemning the attack, and he already closed the book on this chapter in 2010.
Of-course it was correct decision & better to have him on the watchlist before an attack on USA soil than after, as by WTC.