So far as getting excited, you may want to consider that they're in a studio that's probably cold and brutally lit with several dozen other people watching them, which might interfere with arrousal factor. Also, these people are professionals; I'm sure it happens at some times, but when they're being actively directed, hitting marks, and concentrating on their screen performance, not to mention that they're probably doing the same thing repetitively for several hours, it's probably not an issue most of the time.
So far as 'wasting' a hard-on, are you being serious with this question? One thing to consider is that the actors, crew, and production company didn't sign on to produce hardcore �����������, which is quite a different thing than softcore cable stuff in terms of market, distribution, and legality. I'm not even sure what you're really asking here. There's no logic to switching a scripted production over to a hardcore film because one of the actors gets a little turned on, that's just stupid.
So far as nudity, male frontal nudity is allowable to US censors, but showing an erect penis and a flaccid penis are obviously two different things. Just as you only see 'pubic' nudity with women, not a fully esposed and open vulva, an erect penis would probably gather an NC-17 or X rating depending on the context, which is really what matters most. Movies like Shortbus and other sexually explicit films do have real sex or extreme nudity in them, but this often earns an adult market rating and curbs distribution. A pay cable channel might be able to show it, but they run the risk of fines as well as potentially dropped subscriptions and other public backlash. The law is somewhat dynamic about these things, and as I mentioned, context is a huge issue.
But back to the point - softcore ����������� is 'softcore' because there's a market niche for simulated sex and erotica. Not everybody wants to watch hardcore ���� with real sex, and cable channels want the late night ratings generated by explicit sexual programming without the headache of legally and financially hazerdous true �����������. On top of that, you're not going to get the same sorts of actors and production for �����������, as some people just don't want to be associated with hardcore ���� for moral or career reasons.
Hope that helps you see the distinction a little bit.
