They're not, but the integration could attract the wrong sort of attention (both from governments and potential users), and uses engineering effort that could a

They're not, but the integration could attract the wrong sort of attention (both from governments and potential users), and uses engineering effort that could also have spent on whatever someone's favourite missing feature/bug is. Plus, there's just the overall smell of scams that is attached to cryptocurrencies, and people don't want Signal to be (part of) a scam.