Delete causes data loss, and thus is always a big deal. Think about how you use the trash can in your kitchen. Tossing something is an intentional act. Accidentally tossed the $50 Amazon gift certificate in the bin with the junk mail? You might want to retrieve that. If your trash can is an incinerator, you look twice at everything you toss in, or throw a whole bin's worth once a week. Maybe the action is undoable, and maybe it does need to be quick, but neither requires using a GET.
In the mail example, consider non-web mail user agents. They copy the messages from the server to your desk, but have you read those messages? No. Why should the caching of mail by your HTTP user agent indicate that the mail has been read?