A lot of metal containers have linings that are considered safe for humans while sealed. Those same linings are not necessarily safe for other species, and likely become unsafe when exposed to the elements.
I wonder at that use as well, given the following:
Anthropogenic materials used as artificial shells by hermit crabs (n = 386). Plastic shells are the most widely used artificial shells (n = 326, 84.5 %),
followed by metal (n = 22, 5.4 %), glass (n = 22, 5.4 %) and metal and glass (n = 18, 4.7 %)