If you're not getting paid, you're not doing "business".
Yes, there are many other examples of companies having exploratory practices the same way as Spotify. That still does not explain very well why people put up with it.
If a farmer exploits a migrant worker, the migrant worker gets paid enough for shelter and food. He is depending on that job for surviving. When artists are exploited by Spotify, the money they usually receive is nowhere close to paying for shelter or food, so they have to survive by other means anyway. Keeping their music on Spotify or removing it from Spotify doesn't make any difference for their livelihood.
Somewhat different situation than the Spotify case which is arguably mostly an advertising channel while corporate gigs actually pay directly. But creatives of all stripes doing things they'd rather not do so they can live, do what they want to do, and have a sliver of a sliver of a chance to breakout is extremely common.
ghaff already covered why: because Spotify is the distribution channel.
40 years ago, they would've put their music on the radio. Today, they put it on Spotify. Why? Because if they don't, very few people are ever going to even know they exist.
And if you think the fact that Spotify is giving them a terrible deal means they shouldn't engage with them, then I'll point to any of a dozen other examples today of massive corporations profiting excessively off of people who have very little choice but to engage with them. Being able to choose to do otherwise, and remain in the music business, is a very privileged position to be in.