The criteria for his choice seems to be this:
They all continue to convert US players and have the right payment solutions in place and are managed by a group set to stay around for the foreseeable future and defiantly accept US players for March Madness and the triple crown events
So that means expectation of continuation of facing the US.
I am sure that if you list other criteria he will have input about books to match those. He has been in the online sports book industry for a very, very long time and knows it inside out.
Of course if there are reasons to avoid the above mentioned ones affs want to know about it, too.
I have no opinion about sportsbooks, never dealth with them, just pointing out the criteria for his choice.
Re. spam: I got the letter too. It was ill conceived in that it was lacking the usual explanation of how you got on that list and an optout. Simon needs to remedy that.
Re. aff links in this post: I defer to the Professor.