You can! Of course you are allowed to purchase a brand new phone WHENEVER YOU WANT! Go to the store and buy an evo, it will run you $450.
But, phone companies *reward* customers for resigning with them with *discounts* on cell phones. IE, if you get locked into a two year contract with them, you will receive the phone for $200. The whole thing is a money game...as we all know, "time is money". Thus, they give these discounts to get you reeled in and keep you hooked. It's all corporate BS, but in the end, you can indeed purchase a phone whenever you want, you just may not receive the discount you want because they already have you under contract.