OP specifically specifies that this was a "conveyor-belt sushi" restaurant. Those are only "all you can eat" in the sense that you're welcome to pull as many plates as you'd like off of the belt, but each plate is usually priced at one of a few different levels, and you will be charged for the total value of the plates you removed from the belt.
Eg, if yellow plates are $1, blue plates are $2, and orange plates are $3, and you pulled two orange plates, a blue plate and a yellow plate, your total would come out to $9 even if you didn't clear your plates.
Eg, if yellow plates are $1, blue plates are $2, and orange plates are $3, and you pulled two orange plates, a blue plate and a yellow plate, your total would come out to $9 even if you didn't clear your plates.