To be honest neither is slack. They've got push notifications through web service workers. What for would they need aggressive background activity?

I'm not saying this to be pedantic, just trying to nip this entitled vendor mentality in the bud. "We're important so give give give without asking." It's a bit what you see with permissions on mobile: the more famous a company is, the more brazen a permission policy they can get away with. It's only tangentially related actual functionality of the app.

This is not about you personally, by the way, but about vendors. You just shouldn't let them get away with it. Even if they're slack :)