The question is, how badly do they want to catch it?
If it's something only a small fraction of the customers actually bother to do, it doesn't dig into their profits too much.
It effectively acts as another form of market segementation. There is a mostly fixed cost for flying an airplane; the marginal cost per passenger is relatively low. So ideally, you would have a paying passenger in every seat. Some passengers are more price sensitive than others, so you want to have the less price-sensitive passengers pay as much as possible, while still providing ways for the more price sensitive passengers to get fill up seats that are still available after that.
People who are willing to go to the trouble of doing this will happen, but you're still making money off of filling seats with paying customers, they're just acting as slightly more price sensitive passengers.
The airlines fluctuate rates for just this reason; they want people to be able to buy tickets at different prices, so price sensitive passengers can go to a little more trouble to book earlier or take bigger risks by flying standby or waiting for the price to go down if it's not appropriate, while less price sensitive passengers will just buy the ticket whenever convenient.
If this were being seriously abused a lot of the time, or there were automated systems which booked trips like this for you, I could see the airlines doing something about it, but if it's just a few people going out of their way to get a slightly better price, it's probably just considered to be part of the whole ticket pricing game.