The older slower CPUs of the 80's and the early 90's can withstand the heat much better than today's CPU. It's just so darn powerful these days that a heatsink and fan is required, no exceptions. THat is why if you choose to take off the cooling system designed for the CPU, you are risking failure to the CPU.
However, I have an Athlon 64 and my PC shut down when it got too hot. It's making me think that it does have some kind of thermal protection. Well either that or the CPU just stopped functioning and PC just restarted on it's own