Unfortunately at one point in my life it became necessary for me to know about this sort of thing. I'm going to give you two (for now unsourced) statistics:
- The most common factor in successful suicides is "access to lethal methods"
- What survivors of jumping off bridges almost universally say is that on the way down they realised that everything in life can be fixed, apart from the fact they've just jumped off a bridge.
Access to lethal methods
This is a very strange term, it literally means "having some way to kill yourself" but it's stronger than that as it often refers to also having the expertise with which to use said method. Vets have the drugs, farmers have guns, these are two of the highest risk groups in the UK for exactly this reason. Normal people (in the UK) very rarely actually have a reliable accessible method with which to kill themselves. Success rates including attempts and methods USA 2012(pdf) note the effectiveness of firearms attempts (90% success) followed by hanging (83% success) (BMJ)
What your booth does is give everyone access to a way of killing themselves. By providing it you have effectively killed a large number of people who wouldn't otherwise have had a method of doing so.
Jumping off bridges
This is a key statistic, it goes with people taking overdoses then taking themselves to hospital. Far more people attempt suicide than succeed and one of the key factors in survival of an attempt is whether it's possible to stop halfway through. If the method chosen allows the person to abort, often they will.
You're wanting to give them a quick, easy, reliable, painless method, but that's precisely what society works to take away. Suicide level crises are generally brief.
There's another factor to preventing suicide I'm going to mention because I think this question needs it. Most people who are seriously considering suicide have a plan. That plan is fixed, it's an already solved problem that's fixated on, if you take away that method they often won't find another way to do it.
Ask if they have a plan, take away access to that method of suicide and often they won't find another way.
Given the background information above, here's the actual answer to the question
Suicide is legal (in the UK), but only since the suicide act of 1961. Prior to that you could be imprisoned for the attempt or your family could be prosecuted if you succeeded.
By providing the booths, and in light of the information above, the provider could be considered culpable for every single death that occurred as a result of their use. That's around 960,000 additional premature deaths a year in the US alone. When someone is seriously considering suicide, just like any other person who is sick or injured, we have a duty to help them. Handing them a loaded gun with the safety off is not the help they need.
If they step into that booth and they're going to be in there for 4-6 hours, possibly even going to far as to have to book an appointment a couple of weeks in advance, talking to a counselor, putting their affairs in order, leaving messages for their family, having a decent meal, and free to walk away at any point, then perhaps this could be done. Then you could be said to have done what you could to help someone in need and that this is a person who has made a definite clear and conscious decision to die. A simple push a button and it's done for 25p, you'd have some questions to answer, mostly about reckless endangerment.
Suicide Prevention, how to help.