I think 'constantly' is a massive overexaggeration.
Avoiding the catapult doesn't require much focus, and the bombs don't come that often. They're one of four randomly selected phases. Again, the top platform is safe from bombs, so it's a similar situation to Brinstar or Norfair, which to the best of my understanding weren't banned because of their hazards.
I think if PS was legal the only instances of bomb KOs would be people getting hit into them, which happens on Halberd too.
In fact in some circumstances the PS hazards are more fair than the Halberd ones. I remember once I got KOd early by getting clawed while I had been grabbed. At least on PS the bomb would have hit both of us.
Wasn't Pictochat legal at that tourney I went to? The hazards there are far more constant, much more varied and require much more focus.
I personally think all stages with hazards should be banned, but my point is that if hazardous stages are legal, PS should be too.