I would think that usually if some one sees some one about to do this they call the emergency services, so of course it is more likely to be a police/fire officer or paramedic/ambulance technician talking them down. Some emergency personnel are trained specifically to do this.
They stop them to save their life not because they are breaking the law!
My dad...now a retired dog handler for Sussex police talked down a potential suicide bid from beachy head once....then promptly arrested him under the mental health act.
I don't think suicide is illegal in itself but you're obviously not in a good frame of mind if you want to top yourself and you can put others in danger by doing so. By arresting, and then sectioning people, they will hopefully receive the help they need and go on to lead a normal life
i have a friend who recently commited suicide well supposidly and there wasnt even an inquest into his death..so how are his family eva suposed to rest and know for sure how is life finally ended...the police say that an inquest wasnt needed but on the other hand had sum1 have died because ov him im sure there would av been an inquest then!