you have stumbled upon one of the great complexities of smogon ban philosophy, which is banning individual combinations without banning entire abilities/mons. drizzle/ss was an exception, and it's important to understand what the suspect ladder's bans are: garchomp may be legal, but sand veil as a whole (on every single user: gliscor and cacturne as well) is banned. it's not that chomp with sand veil is banned, it's that ALL sand veil is banned. this unearths some serious problems as observed in the chomp/veil discussion thread - why should cacturne lose encore and bullet seed (only legal with sandveil) in lower tiers just because we want garchomp back? is gliscor with sand veil broken enough to be considered an example of "sand veil OP"? what about mons where sand veil is obviously not broken like sandslash?
there is a lot of fear that complex bans (ie banning specific ability or ability+mon combinations) are a slippery slope where once we start allowing things we won't be able to stop, and iirc aldaron intended for the drizzle+ss ban to be an EXCEPTION to the accepted standard (something that many people seem to forget when discussing complex bans). blaziken is a very obvious example of a mon that was not OP until it received its ability but was OP afterwards. however at what point does the complex banning process stop? is excadrill OP without sand rush? is kyogre OP if we allow it into OU at level 50 instead of level 100? moreover many users see it as an injustice to ban the entirety of speed boost just to allow blaziken back into OU/UU since clearly there are other speed boost users (ninjask, sharpedo) that do not even approach being broken.
in other words: it's a matter of ban philosophy, and not a question that can be answered easily or objectively