There are rules. It was part of the C block auction for 700mhz LTE level spectrum.
Google bid like $4 billion that forced a number of FCC rules to be invoked. They did this to stop Verizon from not opening up their access to devices. read about it here:
United States 2008 wireless spectrum auction - Wikipedia, the free encyclopedia
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from the wiki
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Google's specific requests were the adoption of certain policies
Open applications: Consumers should be able to download and utilize any software applications, content, or services they desire;
Open devices: Consumers should be able to utilize a handheld communications device with whatever wireless network they prefer;
Open services: Third parties (resellers) should be able to acquire wireless services from a 700 MHz licensee on a wholesale basis, based on reasonably nondiscriminatory commercial terms; and
Open networks: Third parties (like internet service providers) should be able to interconnect at any technically feasible point in a 700 MHz licensee's wireless network.[9]
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So far, the FCC isn't enforcing it. But Verizon is in no hurry to enable Voice Over (VO) LTE (VoLTE).
It relates to the Nexus 7 issue on Verizon.
Verizon net neutrality controversy: Nexus 7 not allowed on LTE | BGR