"I built my business on top of someone else's product without any gurantee whatsoever of being able to continue in the future, and when it became valuable for them to stop me they did". Doesn't matter if you're a twitter client, a facebook app, a shopify app, a reddit client or whatever, either what you offer is negligible, or you did their research for them and now they can take over.
It's worse than that—within 6 months of them starting to operate Shopify updated the ToS to make it clear that the app they already knew was a grey area was actually formally banned. They received an email that said they wouldn't be shut down right away, which the author took to mean "carry on!" Then Shopify's COO called them personally to tell them to knock it off, and they used a technicality in the phrasing of the…
No, they shouldn't have to play whack-a-mole, I'd argue they shouldn't be allowed to.