a few thoughts on this:
1. To be honest, I never thought McGrath or Hadlee's actions were beautiful, smooth bowling actions. Much like, say, Southee or Hilfenhaus' actions, there is a little bit of imperfection. But I think it's reasonable to allow this level of leeway.
2. Botha's action is bloody ugly, but his doosra has been deemed illegal and his stock delivery is undergoing re-testing, whereas none of Murali's deliveries have ever been deemed illegal. So I don't think it's fair to group these two together.
3. I'm not a biomechanic specialist but I do think that straightening the arm is the distinction between bowling and throwing. Some degree of straightening may occur but I think it's perfectly fair to set an arbitrary boundary.
Arbitrary boundaries are set in terms of length of pitch, size of bat, size of boundaries, size of inner circle, size and weight of ball, LBW laws etc etc. They make things easier to legislate and easier to judge.