I can think of a few reasons why a site owner may stipulate no singles.
It's of no consequence to me. A lot of sites have quirky rules, fair enough, that's their prerogative as it is also my prerogative not to visit certain sites for reasons that others may consider quirky.
The reason that I have a Cali is to give me the broadest possible versatility as to where I go and where I stay. The owners of Searles may possibly be terribly upset when I say that I find Hunstanton to be about as agreeable as piles and their resort to be less welcoming than Chernobyl but then that's my view and if they suddenly made singles welcome with open arms and offered a free night's stay I will still head somewhere else.