The gemara has two approaches as to why the Rabbonon were gozer tu'mah on glass vessels. One approach is that they are comparable to metal vessels since they can be melted down and reformed, just that chazal intentionally implemented some distinctions so that it will be clear that it is only d'rbaonon (so that one is not misled to burn teruma and kodshim after coming into contact with them). The second approach (Rav Ashi) is that they are comparable to earthenware vessels since they are made from sand, just that chazal were slightly more machmir by glass allowing the vessel to become tamei from the outside (since they are transparent, even the outside qualifies as the inside).
Tosafos 16b (d.h. rav ashi) points out that the gemara in Avoda Zara 75b (citing Rav Ashi) compares glass vessels to metal and imposes on them a requirement of tevillah when they are purchased from a goy. Why in the context of tevillah for new vessels does Rav Ashi compare glass to metal to require tevillah, whereas in the context of tu'mah he compares them to metal?
The Ritva writes that really it is similar to both, but in the context of being go'zer on tu'mah, the rabbonon didn't want to spread tu'mah more than necessary so they compared it to earthenware in order to limit the spread of tu'mah. But in the context of the mitzvah of tevilas keilim they were machmir to consider it like metal vessels.
However, Tosafos 16a d.h. ela, writes that even when the gemara compares glass to metal, it isn't completely retracting from it's initial approach to compare them to earthenware. The logic of being able to be melted down and reformed making it similar to metal, isn't sufficient by itself to compel a gezeira of tu'mah. They considered glass similar to earthenware in the primary gezeira, but then made it a little more machmir to receive tu'mah even from the outside since it has some property of metal (able to be melted down). According to this Tosafos, the gemara in Avoda Zara that requires tevila for glass since it has the property of metal in that it can be melted down and reformed, is very difficult. Why is that logic enough to compel a mitzvah of tevillah, and not enough to compel susceptibility to tu'mah?