>maybe it's because the clients they serve (i.e. the people receiving help/services at their nonprofit) are treated as "active members" of their Slack instance?
I don't know anything about slack, but a lot of the saas programs I've supported do something similar where they negotiate a price per 'user' but then during the setup try to get you to start including a bunch of users or change how users are defined to include extra people that are only tangentially related to the day to day operations. One I support, I found out I get charged extra for users of one of the modules beyond the seat charge to already have them in the program.
Or my favourite aspect of this: SaaS that have no facility to avoid charging a per-seat fee for each test user (and of course, each test run needs to create/delete a test user, to test the sign-up flow)
I don't know anything about slack, but a lot of the saas programs I've supported do something similar where they negotiate a price per 'user' but then during the setup try to get you to start including a bunch of users or change how users are defined to include extra people that are only tangentially related to the day to day operations. One I support, I found out I get charged extra for users of one of the modules beyond the seat charge to already have them in the program.