Why is using a HTTP GET to update state on the server in a RESTful call incorrect?

The practical case where you will have a problem is that the HTTP GET is often retried in the event of a failure by the HTTP implementation. So you can in real life get situations where the same GET is received multiple times by the server. If your update is idempotent (which yours is), then there will be no problem, but if it's not idempotent (like adding some value to an amount for example), then you could get multiple (undesired) updates.

HTTP POST is never retried, so you would never have this problem.

If some form of search engine spiders your site it could change your data unintentionally.

This happened in the past with Google's Desktop Search that caused people to lose data because people had implemented delete operations as GETs.