Aluminium is a bit like loctite in that many people wrongly think there is only one type.
Crap aluminium has a tensile strength similar to soap and anyone who has any lathe turning experience will know what I mean.
The better grades of aluminium are very strong indeed and can easily withstand the loads that snapcaps endure.

Dugaboy 1 I find it interesting that an aluminium snapcap rim cannot cope with repeated firing pin impacts but a pencil eraser can.