To answer your direct question, Tether is sold as a fully-backed instrument, so there's major (potentially billions of dollars of) fraud if it's not fully backed. That's an issue because consumers aren't getting what they paid for, i.e. the underlying risk profile is different from what they thought the were buying.

The big difference with Tether is that in traditional banking, fractional reserves are legal, and regulated (i.e. banks are required to prove their reserves, and are required to hold a certain percentage of their deposits).

If there's a run on a consumer bank, and they don't have enough reserves to repay their customers, then FDIC insurance steps in to protect the customers. In this case, there's no protection, so everyone holding tether will be screwed if there's a run.