With illegal drugs, they have already been government certified (class A, B or C) and they are only available on the black market, where there is no incentive to hide what they really are (assuming they are not cut).
With drugs that have not been formally classified, this is legally a gray area, they are technically sold "not for human consumption" and thus marketed "legally" as "bath salts", "plant feeder", "room odorizer" etc... a form of legal plausible deniability for the manufacturer and the shop.
Due to the current legal framework, adding a true ingredients list would only provide additional legal liability to the manufacturer, both alerting the authorities to a new chemical they should look to ban, plus highlighting that the product is indeed "illegal" once the government gets around to updating its list of banned chemicals.
Thus we see the ingredients marked as "ketones", which sounds chemical and technical, but its vague to the point of only describing that molecule has a C=O functional group somewhere inside.
Also in the commercialised "legal high" world, there are no drug patents, only trade secrets. Commercial value comes from scarcity, and if the product itself is not scarce (assuming you know its name), the value comes from the brand name and not disclosing the secret.
The downside is that nobody really knows what they are taking, the scarcity of the tried and tested drugs (which are now all illegal) means only the new and untested drugs are legally available, until human clinical trials show its effective enough to become popular enough for journalists to talk about it, at which point the government bans the effective drug, until the human teenage guinea pigs find something else for the government to ban. It becomes even harder for the new generation to research and stay safe when attempting to get high responsibly.
This is all a dark artefact of the drug licensing laws that try to prohibit rather than regulate recreational drugs. However this would require a law saying you are allowed to openly state that an untested chemical is intended for human consumption and that information regarding its composition and known safety (even if its not been through formal clinical trials) is included and openly stated.