Live shrimp are just that, alive. You can use the fact that they are alive to determine that they are reasonably safe to eat. However frozen shrimp do not have that built-in check.
The FDA and other agencies will "police" commercially available frozen shrimp, but they don't do anything for bait shrimp.
Frozen bait shrimp are usually the dead, dying, or otherwise discarded live bait shrimp. It's not uncommon for a bait shop to sell live shrimp, then take all the dead "floaters" and sell them as frozen shrimp. Again, quite safe for bait, not so good for eating.
Bait shrimp you freeze yourself has a similar issue. If you're eating the live bait shrimp, then you know, for a fact, how old and what condition the shrimp are in. If you happen to have a bait-well full of dead shrimp and freeze it, (again that's fine for bait,) you have no idea why the shrimp died. Moreover, if you "collect" frozen shrimp between trips, you probably didn't bother to mark the freeze date or freeze the shrimp quickly, or a bunch of other things.
In essence, live shrimp you take from the bait-well and boil or try, you know the state of. Frozen (bait) shrimp has been rolling around in the freezer for an unknown amount of time. Well better that one just be bait.
Now you could eat the frozen shrimp if you take great care in freezing it. But again, that's not usually what happens.