As far as why it counts as "doping" — it is probably used by body builders to drop body fat to competition levels. Anecdotally, without chemical help you are hungry all the time at 4-8 %bf.
Also, the common wisdom among runners is that body fat is dead weight and losing it makes you faster and reduces load on your joints. So it may help there too. Cyclists probably see less benefit, but less fat may help acceleration and hill climb ability.
https://www.outsideonline.com/2099881/how-skinny-are-top-tou...
Also running dnp in the summer is not something you want to do. Your body heat goes up like crazy and it's miserable in the summer.
The drug itself is just a dye basically and it will ruin everything it touches.
If you want to know more about DNP search /r/steroids for it. There's endless threds and posts about it's use.
I'll take a lot of stuff, but DNP is the one I won't touch.