This looks like a situation with dissimilar metals and an electrolyte. You might just have a battery.
Canning is generally considered to be a better packaging method for beverages when it comes to flavor preservation, as opposed to bottling. Bottles allow light in, which leads to unwanted flavors developing in the beverage.
A lot of that could be mitigated by finding products that are bottled locally in reusable bottles. I don't know about other countries, but that's become all but illegal in many US jurisdictions.
re: frosting, it really depends on how how you like your beer. If it's an American pale lager like Miller or Budweiser, where the defining feature of the style is that it's meant to have as little flavor as possible, yeah, a frosted glass will help with that. As will a bottle, since bottles are perfect for minimizing your nose's exposure to what you're drinking. If it's an ale that's meant to have a lot of complex flavor and aroma, and you like those things, cellar temperature will bring them out more.
FWIW, that was never true, anyway. It's just that, once upon a time, cheap beer came in cans and expensive beer came in bottles, so it was hard for consumers to make a decent comparison.
When I was in beer judging class, we did an interesting blind taste test among various beers that are available in both bottles and cans. The cans consistently won. It didn't take too long to notice the pattern, either: At least for beers with a more delicate flavor, the bottled version tends to have a certain subtle hint of soggy cardboard that's absent in the canned version.