Cisco basically says you can use Open Source software on your device (the one you're manufacturing) as long as it's not something like GPL3-licensed. Because that would require you to make the software updatable for the user. Their opinion has no bearing on using the likes of OpenWRT or derived AP offerings. None at all.
Anyone who's actually taken a closer look at the relevant FCC regulation (or its equally restrictive ETSI counterpart) will struggle to come up with ways to fully comply with this regulation without locking down the firmware. If you have a WLAN chip that has efuses/internal EEPROM that contain country settings, and if the chip reads them instead of the driver, then all is good. In every other case, it's very difficult.
Of course, neither FCC nor ETSI care about that at all. And manufacturers will probably come up with intentionally lousy ways to lock down their firmware because they still want to sell their products and nobody really wants the default firmware