these drugs will increase your metabolism and burn off calories therefore leading to weight loss. However if you want to keep your muscles you must do high-intensity strength training (weight lifting) at the same time to keep muscle loss to a minimum. If you just sit there and thinking the drugs can make you look like a model you are wrong. "瘦肉精" does not help you grow muscles. it just help you burn off all type of calories fast (like the illegal drug SPEED). There are also a lot of negative side-effects such as increased heart rate etc. associated with the drug. If you want to lost only fat and grow more muscles, you have to exercise (especially lifting heavy weights) anyway, so why not just lift weights and forget about the drug part?
Athletes are competing to win, not for long term health. Athlets will take anything to give them a winning edge. Athlets in general are not necessary the healthest of all, as many of them are plagued with multiple injuries and health problems due to long term abusive use of their physical bodies.
In the US, "瘦肉精" cannot even be giving to pigs or cows, only to horses because horses are not a normal part of human diet. there are many instances of human poisoning when taking meat fed w/ 瘦肉精 in the past so it's now illegal to use it anymore.