Interesting. We might be talking about two different things. When I purchase stamps at my Post Office, I stand in line until a clerk is available. Then I ask the clerk if I may look at the commemorative stamps, since the stamps in the drawers are the stamps which are for purchase.
Are you saying that stamps on display in your Post Office are the actual stamps for purchase? If you see a page of stamps on display which you like, then a clerk will unlock the case and get it for you?
If my Post Office has a place where they hang posters of current commemorative stamps, the posters must be lost somewhere among the clocks, cameras, relevant mailing deadlines, Missing Child Posters and shipping regulations.
Yes, the stamps on display are for purchase. Many hang on a pegboard display and many are special edition collector sets. The display is 4'x8'. You can reach out and take them to a regular clerk or go to a register where stamps, postcards, and shipping supplies are sold.