In my experience of working at startups, I disagree that they probably had a malicious motive when designing this feature. While I think they should allow you to delete the data, the genesis of this experience was probably (1) it took less work to build; and (2) no strong advocacy for user privacy on the product/engineering teams responsible.

As others have mentioned, I think "keep the content and make it read-only" would be the best possible experience, but I also understand (1) that would probably require even more work; and (2) sometimes you need to make business decisions to keep people from unsubscribing.