Maryland apparently forbids restaurants from having open windows, according to an article in TBD cited by the Tales of Two Cities blog.
The article concentrates on Bethesda where many restaurants have large windows that they open onto the street in good weather -- without the screens required by state law. Wordbones points out that state law applies here too. He says the new Stanford Grill in Columbia will need to adapt, but couldn't think of any other Howard County restaurants that have adopted the city-scape design of large, removable windows. Neither can I.