There are, as you say, no US Vendors of complete WHS2011 systems that I know of. The minimum requirements for WHS 2011 are:
- 1.4 GHz x64 processor.
- 2 GB RAM
- At least one 160 GB hard drive.
So if your Atom system meets that requirement, then "Yes". The answer to your last question can only be speculation but I suspect that the volume of sales is not high enough for them to make money.
Phil P.S. If you find my comment helpful or if it answers your question, please mark it as such.