The toilets are not the same as economy. They are more roomy, have designer toiletries and are probably cleaned a lot more by the crew than the economy ones.

But to answer the OP's question, like many have said, it's because of all these amenities that are available for the Business class passengers and so that they don't wait long to be able to use one, Economy passengers are not allowed to use the Business class toilets, unless it's an emergency and the Business class occupancy is not much.