the reason drizzle+ swift swim is banned is because pokemon with swift swim are all water types which means they get stab plus the boost from rain as well as out speeding everything without being locked into a move and everything with chlorophyll is a grass type and wont know fire moves and wont get a stab fire attackLong story short, I think Smogon are a collective bunch of quacks. For instance, their ban on Swift Swim + Drizzle is just stupid if they're not going to ban Chlorophyll + Drought. Their bans often seem like arbitrary decisions to pleasing the whining masses ("Waah! I can't beat a Kingdra with Swift Swim, so it shouldn't be allowed!"), or are just plain ridiculous (Excadrill? Really?)
If the makers of the game didn't want people to use the combos that debuted in Gen V, then they wouldn't, for example, have given Politoed Drizzle in the first place, and just left that apparently too-powerful ability with a Legendary Pokemon (whose banning is much more justifiable due to its vastly superior stats).
Smogon (and websites like it) ruin the fun - no, the point - of multiplayer in Pokemon. So what if you get smashed? make a new team and smash them back! I do think it was an oversight to not include a DW pokemon with Air Lock, but perma-weather can be shut down by other forms of perma-weather, and Thunder and Hurricaine never miss in the rain, which takes care of just about everything that could be 'too powerful' for use.
For these and other ridiculous enforcements, click here: Clauses and Banlists (B/W) - Smogon University