Your probably right, but how would it be easier to pirate it legally?
Currentlt, to pirate Minecraft (when you do it yourself), you need to obtain a copy of the game (which you can legally download from the Minecraft website), and crack it (which is currently rather hard since it is compiled and obfuscated. The fact that it's compiled doesn't make it harder to crack at all, since java can be easily decompiled in a few secounds. It being obfuscated however does make it a lot harder since it takes days, weeks or sometimes even months (depending on skill, luck and amount of time you spend on it) to figure out what each part of the code does and how you can modify it to circumvent the account validation. (assuming you do it yourself, which is more or less legal as long as you don't redistribute the cracked version)
If Minecraft was open-source, then it would be very easy to obtain an unobfuscated copy of the source, which makes you able to find the account validation code much faster so you can disable it in a matter of minutes (assuming you have a decent programmins skill), which means that if you have decent programming skills, you can obtain a cracked copy legally without much effort and without spending a lot of time on it.