He was under NDA but violated it. They reminded him to please not talk in public about NDA-ed stuff and he kept doing it. So now they fired him with a gentle reminder that "it's regrettable that [..] Blake still chose to persistently violate [..] data security policies". And from a purely practical point of view, I believe it doesn't even matter if Lemoine's theory of sentience turns out to be correct or wrong.
Also, we as society have already chosen how to deal with sentient beings, and it's mostly ignorance. There has been a lot of research on what animals can or cannot feel and how they grieve the loss of a family member. Yet we still regularly kill their family members in cruel ways so that we can eat their meat. Why would our society as a whole treat sentient AIs better than a cow or a pig or a chicken?