can't delete folders cause they're write-protected

[AC Question]

I'm trying to delete some music albums/folders from my SD card in a Galaxy S5 using Lollipop. I've tried a few approaches, but keep getting told that it can't be done because they're write-protected. I've tried it via My Files, File Commander and by connecting it to my PC. I've seen some complicated methods involving rooting or taking out the card and sliding switches, etc. But (A) why is it write-protected in the 1st place and (B) why isn't (or is there) a way for a reasonably tech literate person to do this?

Thank you.