Population laws can be a great way to rid many nations of their problems caused primarily due to an unsustainable population though they have to be implemented in the right way.
For example, it has been seen that the poor have a larger number of children than the rich. In addition to the fact that they are usually uneducated and do not understand the methods that can be adopted to control population and what the effects of a large population are, children are also used as a means to earn money by making them start working while they are very young, which destroys their lives.
Methods where the government provides incentives to those that restrict the number of children they have and disincentives to those that have a large number and encourages people to have girl children can help solve a lot of problems.
Population Laws tend to be one of those things that look good on paper but don't work well in the real world. In some countries, there is a real need for population control. We have about 7 billion people on the planet and many countries are simply running out of space. Of course, population laws tend to have some nasty side effects as post 2 mentioned. Take China for example. Male children are valued above females in Chinese society because the male carries on the family name. If the current trends and laws continue, China will face a new population problem. The number of males will far outweigh the number of females and their population will suffer.
Are you talking about a particular population law here? China's one child policy, maybe? Or are you talking about something from literature like the one in Among the Hidden?
In the book, it really does seem as though the population has grown too much and the population law is needed. In the real world, it does not seem as likely that population laws are a good thing. It seems (as it is in the book) very contrary to human rights. We should also note that population growth tends to decline as societies get richer. Perhaps what we should worry about more is getting people richer and better educated.
I'm sure there are governments, or at least officials within some governments, who would say laws controlling the size of families are needed for the economic security of the country now and in the future. From their viewpoint, the laws are very beneficial to the society as a whole. In my opinion, there are other ways to deal with population growth...