For one thing, in many states, particularly in the southern and eastern US where the crabs are mostly harvested, it is the law that you throw back the female crabs. Or, at least, you are restricted on how many you can keep. But it's any wonder why some of them are kept anyway, especially with the egg sacs still intact, considering how the blue crab population has been decimated over a period of decades, never fully recovering.
Scarcity of Blue Crabs
In the Virginia, Maryland and Potomac Rivers, where much of the blue crabs are harvested and live, female crabs and their offspring begin their trek to the Atlantic where the babies are better ensured of survival in salty water. Not many of them make it; due to pollution and predators, many babies don't even make it to adulthood and that precarious future is further endangered if females carrying babies don't even survive to deliver offspring.
A single female can produce up to 8 million eggs and only a small fraction of those eggs will become viable babies. Even their habitat has been threatened, which threatens their survival. In many cases babies don't even have grass to hide from predators, and a recent surge in red drum fish that prey on the babies has served to cut back their numbers after a brief recovery in 2012. To top it off, during this time, a harsh winter also helped to decimate their population, killing 30% of adult crabs in the region. In 2014, the total weight of crabs caught was 37 million pounds, the lowest count ever.
Sixty years ago, the Chesapeake Bay yielded 75% of crabs harvested in the US. Now it yields 35%.
Consequently, states have sought and have implemented restrictions on fishing for blue crabs in the region. They have reduced the number of female blue crabs allowed by 30%, and some have reduced this number even further when populations continued to plummet. States estimate that the female populations would have to grow to 215 million to ease up their restrictions, but populations of females who can spawn have only reached 68.5 million, well below accepted levels.
So, to keep the numbers going upward, there has been a drive to curtail fishing of female crabs who could replenish the population by laying eggs.
The scarcity of blue crabs also drives up the price. It is a simple matter of supply and demand; there is a low, and constantly diminishing, supply of blue crabs, yet a heavy demand. So, there are economic reasons for throwing back those female crabs too; not to mention the obviously ethical and ecological reasons. And the fact that you might not be able to fish and eat them at all in the future if their populations continue to be threatened.