It would be complicated and expensive (yes, WiFi is that complicated), and you likely couldn't sell that open hardware adapter so there's no motivation to make

It would be complicated and expensive (yes, WiFi is that complicated), and you likely couldn't sell that open hardware adapter so there's no motivation to make one. The law states that "an intentional or unintentional radiator must be constructed such that the adjustments of any control that is readily accessible by or intended to be accessible to the user will not cause operation of the device in violation of the regulations" (https://www.law.cornell.edu/cfr/text/47/15.15 https://www.law.cornell.edu/cfr/text/47/15.15 ) So if the manufacturer makes a device where changing the firmware is "readily accessible" to the user and there is an open source firmware available that can circumvent the FCC transmission restrictions (for example, change the power limits or channel limits for wifi physical layer), then that would be grounds for FCC refusing to certify that device, as it is not permitted to make, import or sell general unrestricted transmitters to the general public (there are certain exceptions for licensed operators, ham radio, experimental use by manufacturers etc). It's similar to other clauses that prohibit manufacturers from making it easy for the user to modify the equipment - e.g. 15.203 (https://www.law.cornell.edu/cfr/text/47/15.203 https://www.law.cornell.edu/cfr/text/47/15.203) "the use of a standard antenna jack or electrical connector is prohibited." so that the user can't easily replace the antenna with a different one from what was certified.