I suppose it also depends on what defines a contract. When a person buys a set of plans online, they don't sign any agreements or have any witnesses. They simply put in their credit card number and receive a set of plans in the mail. There may be an agreement between parties but I doubt that qualifies as a legally binding contract.
For instance, if I walk into a bagel shop and the owner tells me that she'll only sell me a bagel if I eat it inside the store. I agree and we finalize our transaction. I now have the bagel but I'm under no obligation to eat it inside the store. I may have broken our agreement, but I'm perfectly free to walk out the door with bagel in hand. Yes I'm now a jerk, but it's perfectly legal. If the shop owner claims that she's actually selling me a license to only eat the bagel inside her shop does that change anything? I'll admit the whole "selling a license to do something" has me a little confused.
If I'm offending anyone please let me know, I'm not trying to undermine the rights of the designers at all. When I saw the "one build" restriction on my plans, it simply got me thinking. This being a philosophical thread on that very same subject, I had to chime in.
And happy new years everyone!