Ok let's get some facts out there.
The OP was not willing to listen to suggestions made to better care for the shark.
It seems he/she is only looking for attention. He has posted several comments leading to the idea that it's all for show and not about the best care for the animal. Other comments lead to the idea that this person doesn't have the knowledge or the means to properly care for it now or construct a tank large enough in the near future.
Yes, sharks lay on the bottom all the time in STILL water. Not even 50% of sharks must stay moving in order to stay alive. It's one of those fish rumors ... like fish only grow to the size of their tank.
As far as why this particular shark needs more room... it can't even turn around without hitting its nose and tail at the same time. That in and of itself will cause injury to its rostrum. Though it will spend a vast amount of time (even in nature) on the bottom, it still needs to move its muscles regularly to keep healthy - that means enough room to get up and swim for extended periods of time. It does not have that right now. It will lead to muscle deterioration, and eventually liver issues - leading to an early death.
Give it to a public aquarium?
WRONG. Public aquariums are flooded with animals just like this. They don't want them or need them buy any means. Public aquariums and zoos have specific animal collection plans (by collection I mean what is on display, not going out and getting some). Just grabbing some random animal from a private owner doesn't fit in with 99.99% of them. Also, all they are doing is saying "Sure, buy something you can't care for, we'll gladly save you" -just like this owner did when he bought it. No doubt the petstore has another one to sell now.
And the best part of this is this thread with loads of sharks and rays from members on MFK:
http://www.monsterfishkeepers.com/forums/showthread.php?t=105011