Idunno why you guys figure it's piracy.
A) You bought Minecraft (right?)
B) Notch is cool (Swedish piratpartiet, etc.), and probably recognizes that (A) is a justification for downloading an old version of the .jar
Per the Terms Of Use on minecraft.net (
https://www.minecraft.net/terms):
The One Major Rule
Do not distribute anything we've made. This includes, but not limited to, the client or the server software for the game. This also includes modified versions of anything we've made. Also, you may not resell any gift codes or licence keys - but of course you can give gift codes as gifts. This is necessary so that we can help stop piracy and fraud - and especially users buying keys that have been fraudulently obtained.
In order to ensure integrity of the game, we need all game downloads to come from a single central source: us. We hope you understand. It's also important for us that 3rd party tools/services don't seem "too official" as we can't guarantee their quality. Make sure to read through our brand guidelines. If you wish to make something pertaining to anything we've made we're humbled, but please make sure that it can't be interpreted as being official. Do not make commercial use of anything we've made.
Regardless of the legal issues behind it (the HLP server is based in the USA and, for good or for bad, is subject to US Law), because Notch is who he is and how he generally treats the user base, respecting their wishes is something that I personally have no problems with.
As I understand it, because of the legal situation, and the fact that the minecraft jar's are copyrighted code from Mojang, sharing the jars falls under the umbrella of copyright infringement and piracy, therefore is not allowed.
I wish that Mojang would make it easier to download older versions of the game, but as their primary focus is still single player, I don't see it happening anytime soon. Besides, there are a few ways to download older versions or restore them, you just have to look around (*cough*ivementionedafewearlierinthethread*cough*)