One individual home router operating illegally won't do much to increase the noise floor, but each router operating illegally adds that much more to the background noise and makes it that much more difficult for the weather radar to function properly. So yes, they were doing harm to airport operations. It's just like light pollution -- one light won't block out the stars, but a million will.

They were causing interference because they could be (and were) modified to behave in a way that violated FCC regulations. So the FCC made another regulation that said devices should not be modifiable in that way. This seems totally sensible to me. TP-Link violated this rule--not just the "letter" of it.

Also, nobody has banned OpenWRT. If you read the consent decree, you'll see quite the opposite.