Locking down would encourage to violate the regulations
Background information: WiFi, in the 5GHz range is shared with radars, which are for instance in use in airports. In practice, in some channels the hardware/software need to do active radar avoidance: If they see a radar beam, they have about 10 seconds to change channel. That radar avoidance is called DFS.
A discussion about WiFi DFS has pointers to ways in which locking down the firmware actually encourage people to violate the regulations. For instance in this article about DFS there is the following:
If you vendor is unable to fix the issue, it may be worth trying an alternative vendor. Though this may seem extreme, I have seen huge variations between vendors and their susceptibility to DFS false positives. A limited-scope proof of concept costs little to deploy and can provide amazing leverage with your existing vendor…
Also some old proprietary firmwares for WiFi access point typically used in mesh networks have issues with radar detection. While vendors might improve things in later version it's impossible for them to support hardware indefinitely. The only way to solve the issue would be with free software, for instance by installing a free software firmware like LibreCMC which is maintained by the free software community as long as possible.
It means that in some cases, it is impossible to fix that issue with nonfree software, which could push people to violate the regulation not to have to buy new hardware. If the software is signed, people might not be able to install free software to fix the issue.
In contrast free software drivers like ath9k are pretty good with DFS and can probably be tuned if necessary.