Most modern / renovated hospitals (last ~10-15 years) have designed their isolation rooms to be negative pressure anyway and standard patient rooms also operate under negative pressure because of how airhandling is setup (albeit a very light negative pressure)
Perhaps the US is different(I work in IPC in Canada), but most isolation rooms are not negative pressure capable. It is very expensive to design a room to accommodate negative pressure (separate air handling, HEPA filtered exhaust, sealing the room to the true ceiling). Most isolation rooms are designed to handle contact isolation patients, and therefore no negative pressure is needed.
I'm mostly familiar with ER design so I may be off, as far as what "normal" rooms are like. But from my recollection there was a push after 9/11 to move any new isolation rooms to negative pressure and there were subsidies in place to encourage that. There was also a push to redo airhandling for standard rooms to approximate negative pressure but with less stringent scrubbing.
You could be correct, I'm not familiar with post 9/11 IPC planning in the States, however I highly doubt that all isolation rooms in the USA are negative pressure. I assume the post 9/11 push was for all hospitals in urban settings to have at least negative pressure isolation room (very different from an isolation room).
