They could be waiting until it becomes a big issue

I guess I don't see that as a problem if its causing a big issue.

Let me throw it back to you: If you were providing a service and a third party client was using your resources and causing a "big issue" like you stated, would you not want to remediate the problem? Lets say you introduced a new feature, but it doesn't work for 15% of your user base because they're using an outdated third party client that may not get fixed for another year or two - if ever. What would you do?

Here's another example, lets say someone develops a client that lets you upload significantly bigger files and has an aggressive retry rate that as more people start using your client, it starts increasing the hardware requirements for your infrastructure. Do you just say "oh well", suck it up and deal with having to stand up more infrastructure due to the third party client doing things you didn't expect? Is that reasonable?