A while back fellow AB-er Steve.5 and I had a debate about Muralitharan's action and we agreed it was very suspect. The rules certainly seem to have been amended to suit certain individuals' apparent physiological difficulties. I may be wrong but as far as I can remember the original law contained nothing about a fifteen degree allowance.
One thing that does strike me is this: all those investigated have been from the sub-continent. This comes as no surprise. I have visited India, Sri Lanka Bangladesh and the Maldives (where a large number of people from the mainland and Sri Lanka live). I've been to those places a number of times. I've watched the youngsters play cricket on the beach or in the town squares. They usually play with a tennis ball and an orange box for the wicket. But they all, almost without exception, throw the ball when bowling.
A coincidence perhaps, or a habit some of them find hard to get out of?