Hacking is just generally frowned upon by most of the community. Cheating is, well, cheating. And nobody likes it (except really extreme cheaters). It's like if you won a trophy for winning a tournament but you didn't even win, and didn't earn the trophy. It would give most people a feeling of dissatisfaction. So when people see an outrageous Pokemon (like shiny Rayquaza), they're skeptical of its legitimacy.
Honestly I see no difference between event Pokemon with special moves and hacked Pokemon with special moves and that mimic events. The only difference is that one is official. Otherwise, they're both identical. In fact, whenever I get an event Pokemon, I feel a similar feeling of dissatisfaction knowing that I didn't do anything for that event. I didn't even have to go anywhere, all I had to do was open up my 3DS and select Mystery Gift.
Anyway, hacked Pokemon are counterfeit and pretty much disliked. It's understandable. It's like receiving a really special prize for doing nothing. That's pretty much the reason everyone hates them.
Also they're banned from official tournaments, so if you get caught playing with one you're disqualified.
Hope I helped you understand it a bit more :)