There is the legal concept of Mens Rea which has to do with the mental state of the person committing the act. And I think that applies in this case. Archeology has generally been about learning and providing knowledge of previous cultures. While the methods, mindset and actions of 18th and early 19th century treasure hunters left a lot to be desired, some of them did make some reasonable attempt at documenting their finds and preserving the context to provide that knowledge. Modern archeologists go to painstaking lengths to properly document finds and preserve as much knowledge as possible from finds. Grave robbers do none of this. Their motivations generally revolve around personal gain and they will destroy any context and knowledge in their attempt to make money.

Consider your own reading on the Valley of the Kings. Where did all of the information we have on the Pharaohs in those tombs come from? It's from the work of the archeologists documenting everything found in those tombs. While there is certainly an argument for leaving things in the same state they were found in, that also means that the artifacts will continue to deteriorate and any further knowledge which might be gleaned from them will be lost. Sending artifacts to a museum isn't all about putting them in cases for people to gawk at. It also means that actions are taken to preserve those artifacts and maintain them for observation and study in the future. Sometimes this does cause damage. Again, 18th and early 19th century preservation was often just as, if not more damaging than leaving those artifacts in-sutu. But again, the intention was to preserve, not enrich.

So, that's how I would draw the line, based on the reason and methods used for the removal of grave goods. Is it done with the intention for the furtherance of knoweldge of previous cultures? Or, is it just done to enrich someone? And is the work being done using the current understanding and methods to best capture and preserve that knowledge for future generations?