The barrier reef is a very complex eco-system. The turtle grass beds are very much a part of this system, they hold the land together and protect the island from being pulled out to sea, also make as a fish hatchery for the fish and other sea life of any tropical destination.
Try to remember, we are the guests to the sealife, not them the guests to us. Dredging kills off many creatures and pulls land out to sea, so they try not to do it. In some cases a square patch as is described above out a ways from shorline is feasible but doesnt last long.
Belize is a snorkler and divers paradise, not the greatest for shoreline swimming. Throw on a snorkel and mask and swim around on the top of the water, so you can see all the wonderful life underwater that Belize has. There are lots of things you could step on, rays for instance crabs and other delicate sea life, that wouldnt enjoy your foot so much on its back. Have fun and hope that helps you to understand why there is so much turtle grass inside the reef system.