TriNet considers cancer diagnosis bad business?

Why did TriNet send a letter to terminate its relationship with my company the month after we informed them one of my employees was diagnosed with cancer and would require expensive medical treatments? It's curious because (1) I have spoken with two other companies who had the same experience, and (2) at the same time, TriNet's aggressive but clueless salespeople were contacting me to sell us on their PEO unaware we were already a customer. (Yes, I can provide documentation and references.)