Minecraft is easy to pirate...but why? Well to answer you: there is 0 effort to pirate it, download the client.jar from mojang official APIs (which is totally FREE) and run the jar without using any valid session ID, done! What...that's too easy, were is the catch? Well you can't have a skin, you can't play minecraft servers that don't allow un-authenticated users and anyone can use the same user without any password
Is that really piracy? I mean i am using a free thing, not messing with microslop shit and servers - yes i did read mojang eula and no- it's not against it.
So i wonder: can we not call it piracy? Dunno...i leave the words to y'all!

Edit:
i don't think it's piracy because when you buy minecraft they give you an account to login in their auth servers, but you don't even need to use the auth servers to access the game, just to use realms, access servers that don't allow non authenticated users etc etc