How often do you think it can be expected that your program will have the whole of the computer to (ab)use? How often do you think the computer doesn't have several other tasks to run?

Programs should not bother about limiting their CPUs/CPU time usage. Actually it is quite the opposite, they should use as many (concurrent) CPU resources as possible in order to finish sooner.

It is the operative system who has the responsibility to ensure that some program does not degrade the performance of any other application running concurrently by just creating too many threads or any other mean.