They could read your text, provide their service and then discard it. Instead, they store it indefinitely. He states this explicitly right here https://twitter.

They could read your text, provide their service and then discard it. Instead, they store it indefinitely. He states this explicitly right here https://twitter.com/sebmck/status/1104132997110878208 https://twitter.com/sebmck/status/1104132997110878208