You're more likely to be eating bogus fish
There's a big problem with grocery stores and crappy sushi bars mislabeling fish. This is more likely a supply chain problem than any unscrupulousness on the part of of the seller, but respected sushi joints won't get burned by their suppliers.
Grocery store sushi is oxidized
The point of cutting sushi just before serving is that meat oxidizes when exposed to air, and the flavor deflates. No amount of loosely fitting plastic cover can keep that fish from turning sour once it's been cut.
They're not starting from the whole fish
Cheaper places are starting from pre-cut fillets, and there's no telling if there is something about the whole fish that might cause a more experienced chef to discard it.
Less flavorful fish are more common
Salmon and yellowtail are big sellers at grocery stores because they're fattier, less flavorful cuts whose melt-in-your-mouth qualities mix well with sweeter sauces.