Does anyone know why a Canadian iPhone 4s for Virgin Mobile would not work in USA?

Call Virgin Mobile and ask them if it should work, and I would also ask if there will be international fees. That is assuming you have a phone around that is working. American pay phones you should be able to dial 800 numbers without needing any coins. Most hotels it is free to dial 800 numbers, but you should double check where you are staying if you are in a hotel.

I thought America uses both frequencies, but I could easily be wrong, so I will be following to see the answers.

The phone may need to approved for International use. When I travel out of the USA I have to contact Verizon Wireless first. Check for roaming charges too, it may get expensive.