I wouldn't necessarily say that because there are tons more shiny Pokemon floating around, people must be cheating to get them.
Shiny Pokemon are becoming more and more common for a variety of reasons. The series introduced several features to increase the encounter rates, such as breeding with international parents, chaining (4th gen only), and using the shiny charm (B2W2 only). The series made shiny Pokemon more well-known. For example, one can catch a shiny Haxorus in B2W2 or one had the opportunity to receive a shiny Larvitar if one attended the championships. Cheat devices loaded with the cheat codes are easily available in stores.
The implementation of Wi-fi trading obviously will spread more shiny Pokemon around, as traders highly value said Pokemon. However, realize that internet connectivity itself can increase the amount of shiny Pokemon showing up. Fans create websites and discussion boards and will spread their shiny-hunting tips and stories. If one reads a post about someone receiving a shiny Oshawott or one watches a YouTube video of someone running into a shiny Beldum, some viewers are influenced enough to hunt for their own shiny Pokemon, legitimately increasing the pool of shiny Pokemon. Humans are very competitive...
Another reason why there are so many shiny Pokemon is because of RNG manipulation. BW has the easiest RNG mechanics to abuse in the entire Pokemon series. What I'm basically saying is that if you download a computer program called the RNG Reporter, which predicts when and where the game generates a shiny Pokemon, and follow an RNG abuse guide, you'll be able to advance the frame / do other necessary things enough to walk right into the shiny. You may need several attempts, but a 1/5 tries is better than 1/8192 tries, right? RNG abuse will also legitimately increase the pool of shiny Pokemon. Some spend lots of time playing these games, haha.
Cheated Pokemon can get on the GTS; some are blocked, but many Pokemon aren't.
Anywaaaays, I did find my first Unova shiny 4 hours after buying Black. :)