The National gardens are for display and information.  If they were to dead head the gardens, it would remove important information.  How many blooms did this iris produce?  If I'm torn between voting for two similiar irises it helps if I know which one produces the most buds.  Of course, there are many other things I'd want to know too, but this is how people know bud count.  Same is true for seedlings on a show bench.