Personally it would annoy me too. The curtain does create the slightly more "intimate" environment you pay extra for in a business class seat. I don't have massive issue with some passengers coming to use the bathroom if they can't get to the back of the plane, but being completely frank, the business class tickets often subsidise the cheaper tickets for the economy passengers.
When I fly economy (and bear in mind this is most of the time), I use the bathrooms relevant. I don't see any issue with a curtain being drawn. Not having a curtain drawn would seem odd, to me.
I am surprised by the mockery of the OP here - if you fly business or first you should, rightly so, expect that extra service and comfort.