I remember back in the day Heroku had a huge store of integrations that you could just turn on with a click and they worked like that. You'd get a New Relic account that was tailored to dyno performance and you accessed it via your Heroku dashboard.
It became "the way" a lot of these PaaS systems operated and I'm sure the goal was to get some percentage once you increased your usage from the free tier, which makes sense for the PaaS partner.
For sure, revshare is standard on those partnerships.
Fun(ny) fact: all the companies that started out on Heroku back then are still locked into those Heroku-captive tenant accounts on those partners, because contractually, the partner is not allowed to transition such an account to direct billing. One company I've worked with has had all their infra moved off Heroku for almost a decade, but their Sendgrid account, which has hundreds of subtenants that each have custom domains configured, still can only be logged into via Heroku. They'd have to rebuild that whole thing from scratch (including make all their customers redo DNS validation) to move to a real sendgrid account.
I'm sure Heroku earns Salesforce a really healthy revenue stream based on this.
It became "the way" a lot of these PaaS systems operated and I'm sure the goal was to get some percentage once you increased your usage from the free tier, which makes sense for the PaaS partner.