[volemo]

Because you probably (or more likely your employer) pay Slack.

[statictype]

The free version doesn’t show ads. Come on.

[konha]

The free version is a loss leader to build mindshare amongst people who are (later) in a position to buy licenses.

[arccy]

They do try to upsell you hard though... 1st party ads are still ads

[parthdesai]

How is that an ad? You get limited set of features for free, if you want to use other features, they tell you it's only available in paid version.

[1123581321]

They advertise in the sidebar, offer discounts for the first month, occasional free trials of Pro features. You don’t have to try to use a Pro feature to get the sales pitch.