Translation:We are putting the plates on the table.
It's in the accusative because motion is implied.
i would say, grammatically yes, but realistically not so much...i have a German friend (who lives in Germany, native speaker) and she says it doesn't matter to her in conversation because she can still understand what i mean.
BUT, i think it would be best practice to say it correctly! this is what i will always attempt to do.
Roughly speaking, "stellen" implies vertical while "legen" implies horizontal. "Ich stelle den Stuhl auf den Boden", for example, means that you're putting it on its legs. "Ich lege den Stuhl auf den Boden" means that you're laying it down horizontally. I realize that doesn't really explain why you use "stellen" with plates, though; I guess it's just idiomatic.