Elizabeth
the truth is seldom pure and never simple.
I think you are correct in saying that lists of names etc are 'public domain' but I believe that later copyright law has stepped in to protect the effort of those who have dug out information which was not otherwise readily available.

I have placed a large amount of scanned material into readily available public use (I have more that which comes within the 70 year copyright rule) - it has taken a fair amount of my time thus I would be significantly upset by someone taking the material and passing it off as their own (as once happened re my Manx parish pages!) Now for many who digout the material (often at significant personal/financial cost) they may be very willing to make it public as in a book or on a web-page but this should not allow wholesale theft for republication either for gain (as you claim this commercial company does) or for personal advancement - they should have the right to control some use (the 'moral right' of authors enshrined in most European copyright laws).
Ancestry.com no doubt provide a useful service (I dont know) but they have to pay staff, web-hosts etc (as eg I have to do once I leave the Univ site) Nothing comes for free in this world except death and the tax demand. If you made use of their site to save you the time (and cost) of digging out the info yourself then you owe them whatever contractual fee you agreed to - this 'fee' would have been in the legal small print when you accessed the site - if this fee is 'no subsequent re-publication' then legally you agreed to it - if you didnt agree then you should not have used the information.