Joe, it's not a "seafood restaurant," it's a crab house.
It's a treasure because it's a real crab house -- fat, heavy crabs liberally seasoned with JO and dumped piping hot on your table, served with lots of very cold beer. You're right, anything off the menu sucks. But that's not what it's about. I completely disagree that a good crab house needs a good soup, a good crab cake, or least of all good onion rings.
The old Angelina's (which got raves from you) had what was arguably Maryland's best crab cake, but the rest of the menu was barely edible. Canned veggies? Yep. Fries and onion rings from freezer bag to fryer to table? Yep. They did one thing very well, and sucked at everything else.
Yet that was far less defensible than what Sue Island offers. Angelina's pretended to be a full restaurant. The Sue Island menu is basically there for non-crab eaters who end up accompanying a group. One could easily argue that it would be nice if they did a better job accommodating those unfortunate folks, whose lives are already sadly empty without the further insult of bagged chicken fingers, frozen pizza or greasy onion rings. But that doesn't affect the real mission of the place, which is great crabs and cold beer outdoors by the water. And they do a perfect job of that.