How was the discovery of Tutankhamun's tomb and removing its contents not considered grave robbing?
First of all, Tutankhamen's tomb was previously robbed by ancient thieves intent on carrying away the riches which were buried with him. His tomb was largely intact because he died at a young age (eighteen) and in an untimely manner, so he was buried in a tomb prepared for another. This serendipitous event left his tomb mostly undisturbed.
Those who began removing artifacts from his tomb after its discovery largely have used them for scientific and historic study. As noted in a previous post, the physical remains were subjected to abuse, probably as a result of over-eager scientists and archaeologists. It is not grave robbing because those who discovered it did not convert its contents to their own use; rather they became part of the historical record. Since he was a reigning sovereign of Egypt and since his remains have provided a wealth of information about Egyptian culture, everyone has benefited, not just the discoverers. For that reason, a suggestion of grave robbing does not lie. For the purists, one might argue that his grave was desecrated; however beyond that, no "robbery" occurred.