I think it is incumbent on anyone visiting another country to know what the rules are...I would never go to Russia, for example, without reading up on their rules for visitors first. I think with the UK, the rest of Western Europe, Canada, and the Caribbean, Americans (not all Americans obviously, but a lot of them) tend to just hop on a plane and not worry about it too much. That attitude isn't the best, honestly and people should be more proactive about these things.
That being said, I totally agree that it is unintuitive that the definition of "work" includes volunteering or the definition of "study" includes taking some casual dance classes. Things like that should be spelled out for people in an easy to read format, but I don't think it's necessary to spoon feed it to them- a simple page on UKBA's website or UKVisas under the visitors section that explains some of those tricky nuances would be adequate, IMO. At that point it's up to personal responsibility for someone to read what is out there, and if they don't, well that's unfortunate.
Producing a webpage for non-visa national visitors would be easy as pie, but I'm not sure of what the chances of getting UKBA to add something like that are.