How do I get my iphone 4s & 5s carrier unlocked?

I read the section that said to ask the carrier to unlock it. I called Verizon they said they can unlock it for global only. Which means I can only use it on a global prepaid card. I'm in the US. They said the 5s is already unlocked for global. I want to use a prepaid carrier (straight talk). I really don't understand why it's so hard to get something unlocked. I don't want to have to pay several hundred dollars to get some company to unlock it especially when it's probably just a scam. I don't want to jailbreak it either. We should be allowed to use whatever company we want.