It's the other way around. Why does Facebook think it has the right to remove years of their users' contributions?
As the years march on, I think we'll see more and more data protection laws in favor of users rather than companies. Data is the new uranium, not the new oil. It needs to be protected and taken seriously.
You can argue that it's Facebook's site and that they can do whatever they want. I argue that they cause material harm to users by removing years worth of social interactions among their family and friends. Emotional harm is still harm, and it should be treated as such.
In other words, it's a shame that they didn't catch this use case sooner, but that's really not the users' problem. The fact is, users were doing this. And now that they've been doing this, it's unethical to wipe all of their conversations just because you feel you can.
The largest companies are mostly monopolies. When there's nowhere else to go to have a conversation with the people you care about, what are you supposed to do when companies suddenly tell you that your conversations are violating their terms of service?