The flipside of requiring a provider to submit paperwork to terminate your service, is the situation where in order to setup a new server with DO you'd have to file paperwork yourself. Can you imagine sending a paper letter every time you need to spin up a VM?
Which one do you prefer? paperwork in both directions or no paperwork?
I used to work for a SaaS firm. Every once in awhile the main office would receive a written and signed letters from (primarily) German users when they wanted to terminate their service. (We accepted that after confirmation using an authenticated email address, of course.)
Email is legal in all countries, as far as I'm aware.