Steve,
Don't forget that the US ADS-B system is rather different to that elsewhere. They have ADS-B on 1090MHz that PAW can receive but they also have ADS-B (called UAT) on 978MHz as well, that PAW will not receive. And all the extra service broadcasts (Weather, FIS, etc) are only on that UAT frequency. Plus, as PAW was not designed with UAT in mind, it wouldn't know what to do with the extra data even if it saw it.
There is a ground station rebroadcast of 1090MHz traffic to 978MHz receivers and vice versa, but as I understand it this only happens if you are broadcasting ADS-B Out yourself - I.e. Detection by the ground station of your broadcast on one frequency ( or if you are lucky, another a/c nearby) will trigger the ground station rebroadcast of traffic on the other frequency.
So, if you go flying in the US with a PAW but with no ADS-B Out on 1090MHz or 978MHz your PAW will only detect any nearby 1090 MHz ADS-B traffic and your PAW will only receive 1090MHz rebroadcast of 978MHz traffic if there is another 1090MHz ADS-B broadcaster near enough to you to trigger the ground station rebroadcasts!
Hope that makes sense!!!
Steve