Wikipedia bans Tor editors because editors who use Tor are overwhelmingly doing so to avoid bans. I'm sure there are a subset of Tor editors who are good-faith contributors, but, just like with HN, a bad-faith majority has ruined it for them.
Banned entirely or just from editing? Perhaps websites (especially ones that value intellectual freedom such as Wikipedia and Hacker news) could allow certain IP addresses that announce they run exit nodes the capability to have certain accounts (those of users who have direct access to the network) whitelisted.