I've never used AWS before so you can consider this a "fresh perspective". In defense of AWS, the warning starts with "This action creates a copy of the object". In defense of the author, "Edit storage class" is a pretty misleading name for the operation (how about "Copy with new storage class?"), and perhaps these "warnings" show up under other more routine operations frequently enough --- and unimportantly-enough to cause warning-fatigue.

However, given the nature of cloud services in general (they have a strong $$$ motivation to nickel-and-dime you for every little thing they can) and especially AWS' reputation for doing that, I likely would not have made this mistake. Then again, I'm also someone who carefully reads all available information --- especially whenever it's something I'm paying for.