there is no such thing as a “legal high”. Its an obsolete marketing term dreamed up by the sellers of these chemicals – they are simply recreational drugs that the authorities have not yet round to getting classified.
The concept of taking drugs for recreational (instead of medical) usage in this and many other countries is classed as a criminal act, although there are obvious anomalies as the law lags behind science.
A “legal highs stall” potentially breaks a number of laws (not just drugs laws but health and safety laws about chemicals etc) and puts your event at greater risk of being forcibly entered by Police and other government officers, and closed down.
Whilst the crimes committed aren’t major, its the same as for instance committing minor traffic violations on a bicycle. In most cases you will get away with it, but as soon as someone complains or it affects others (such as if you aggressively rode at 20mph through a pedestrianised shopping centre and were subsequently involved in a collision) the authorities will take action. and this is exactly what has gone on – people on “legal” highs have done stupid things, hurt themselves and others and have thus made the authorities react.
