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

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

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

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

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.

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.

>Alongside of private messaging, people were saying they wanted to hear more about topics, teams and organizations across WhatsApp.”

That was in response to this.