Bath salts are not a specific kind of drug. They are a group of legal alternatives to illegal drugs like meth and cocaine. They are not legal to be sold for human consumption, they are just not criminalised in some countries because they are relatively new.
They only exist because other drugs are illegal, these legal alternatives will always exist and they will always have terrible side effects until other drugs are decriminalised. The war on drugs is pointless and this is a prime example of why.
Smoking is a drug, it just kills you slowly enough that the government don't give a crap. By the time you die from cancer you've probably been working and paying taxes your whole life (as well as tax on the cigarettes) so what do they care?
There should be no such thing as an illegal drug. Because stupid laws spawn stupid work arounds, if I really wanted some heroin I could get it right now. It's only a phone call away, it's even easier than going to the shop for a packet of cigarettes or a bottle of vodka, and I wouldn't get ID'd either.
The whole "cannabis is a gateway drug" argument is entirely invalid and unfounded. If beer were illegal, you would go to a drug dealer for it. The drug dealer doesn't care about the law, and also sells cocaine, you might get pissed and then he could persuade you to try some coke. Does this make beer a gateway drug? No. You don't come into contact with cocaine, heroin, meth etc. when going to the off-licence, because the off-license doesn't sell cocaine, because it's illegal. If they sold weed in shops you wouldn't need to go to a dealer who sells harder drugs and therefore it would not be a gateway. Weed is only a gateway in the sense that you are more likely to come into contact with harder illegal drugs because it is also illegal.
EDIT FOR PEOPLE WHO ARE UNAWARE OF BATH SALTS:
Bath salts are not actually bath salts you would use in your bath, in fact they stink like a urinal. "Bath salts" and "plant food" are nicknames for a variety of amphetamine drugs. Before they were criminalised (in the UK anyway, they are still legal elsewhere) they were sold as bath salts and plant food to avoid them being tested and rejected for being unsafe for human consumption. Not many people refer to them by these names any more in the UK, but the name has stuck in the US, mainly because they have been in the newspapers under the name "bath salts" whereas when they became a problem in the UK, papers used more specific names, sometimes the name of the chemical compound. The American media, government and police spread the name "bath salts" and demonised it, but in the process lumped other completely unrelated drugs in under the name "bath salts" as well as not giving the public any real knowledge. If a dealer offers you some mephedrone or white lightning do you know what that is? You haven't heard the name in the news, so it can't be that bad right? Try some, see if it's any good. Then bam! You're eating someone's face off in the middle of a duel carriageway.