Good question. You can certainly proxy SIP. But what’s notable here is that the traffic comes directly from organizations that should be secure: banks, infrastructure, governments. For a proxy to explain it, the proxy exit node would have to be running inside those institutions, on their networks. So from a security standpoint it doesn’t really matter whether a botnet is running inside the company or the company is being used as a proxy hop. Either way, a machine on their network has been compromised.

(And your P.S. is spot on. The attacker bears none of the cost; the clean IP reputation and the blame land on the compromised third party.)