I don't think it's ignorance at all; I think they're feigning ignorance. Just completely denying it. Because anything less than that would be blown up by the media. If they admit that yeah, sometimes they were stoned in the studio, the media go nuts ("Pink Floyd Admit to Being High in the Studio"). If they admit that they're heard from fans that their music is great to listen to while stoned, the media go nuts ("Pink Floyd Admit that Their Music and Drugs Go Together"). No one would be surprised by this, but Pink Floyd, despite being rockers, are proper English gentlemen from good homes and well-to-do families. It seems to me that they made some kind of decision from the beginning to just deny any kind of link to drugs, because drugs are still illegal, and that's a can of worms that they will not open. And yeah, after the thing with Syd, that just strengthened their resolve.
So let the rest of the world make the association, let a large percentage of fans make noise about it, it's no secret; but officially, nope, they just don't get it.