The immigration office in Pai is a substation of the Maehongson province immigration office, the HQ being in Maehongson town. I haven't heard anything about this, but if it's true then that's the reason for it.
There are no restricted countries for US citizens or green card holders, not sure about the b1 visa holders. There were some rumors about banning travel from India, but I don't think that's happened.
Covid test is the only entry restriction into the US. Some states had attempted to mandate quarantine, but I don't believe there are any currently. I flew into the US from South America a few weeks ago and didn't have any issues.