At the moment there is a limit of 50 MH/s per worker. If someone has doubts about this - please feel free to test it. One can rent a rig (for example from betarigs) that has hashrate above 50 MH/s and try to mine with it. You will see that this worker will give you around 50 MH/s.
Miners that have total hashrate above 50 MH/s are using multiple workers and that is why you see some miners with 300 MH/s and above. We will let the pools run as is for now. We are considering implementing limits per IP. We might implement per IP limits in the next 24 hours.