Thought i might give you the Australian perspective:
I volunteer with St John Ambulance, we do care at sporting events, concerts, public events, disasters, and when it is required.
Our training is very comprehensive, so we normally carry oxy viva's, heartstarts, penthrane and 'laughing' gas (enternox (sp?)), among lots of other pieces of equipment. On duty or not, we can more or less do anything up to the extent of our training without having to worry about the law.
If we try to perform surgery, obviously we have then stepped into the wrong!
Becuase of this, we don't carry BP Cuffs or Scopes, on the grounds that we can't be trained enough to know how to interpret the results to the same degree as a medical professional, i.e. a doc or a nurse.
Carrying things that we don't have the 'training' to use is not permitted. Even though i can use a BP cuff for example, there is very little we can add in regards to different situations under Australian law.
The vast vast vast majority of casulaties we treat are very pleased for the attention, and do not complain or file legal action. I believe (Although i may be wrong), that no Australian volunteer has ever been sued for his/her actions. Part of that is that we don't do the wrong thing - and we don't exceed our training.
We don't really have any EMT equivalent here, only what you can do through volunteer organisations. We do things like advanced rescusitation, a lot of psychological and operations stuff, triage, some rescue things, as well as basic injuries. At least we don't get sued
Thought some of you might like some insight into how it works in a less litigious society. (And for the record, my pupil pen uses button cells (grr), and we can't carry EpiPen as it is considered prescription only)