Our society is built on trust. Do you test the water from the city every time you drink it? Etc. Days like today show that, yes, the kernel team is doing their job.
How about -you- prove that they -aren't- doing their job?
"Suppose you use some company's service and they refuse to cooperate in regards to pentesting ... Is that unethical?"
Yes. You are doing it without their consent. It is unethical. Just because you think you are morally justified in doing something without someone's consent does not mean that it is not unethical. Just because you think the overall end result will be good does not mean that the current action is ethical.
"Yes, but only because that's illegal, not because it is unethical."
This is very pedantic. It's both illegal and unethical. How would you like if it you had a business and random people came by and picked locks, etc, in the "name of security"? That makes zero sense. It's not your prerogative to make other people more secure. If they are insecure and don't want to test it, then it's their own fault when a malicious actor comes in.
"Again, there's nothing morally wrong with "stealing" someone's wallet and then giving it back to them"
Yes, it is morally wrong. In that scenario, you -are- the pickpocket. This is a serious boundary that is being crossed. You are not their parent. You are not their caretaker or guardian. You are not considering their consent -at all-. You have no right to "teach people lessons" just because you feel like you are okay with doing that. If you did that to me I would not hang out with you ever again, and let people know that you might randomly take their stuff or cross boundaries for "ideological reasons".
"Because the risk of running into a physical altercation is quite low, as is the risk of getting arrested. "
This is admission that you know what you're doing is wrong, and the only reason you do it digitally is because it's more difficult to receive consequences for it.
I strongly urge you to start considering consent of other people before taking actions. You can voice your concerns, but things like taking a wallet or picking a lock is crossing the line. Either they will take the advice or not, but you cannot force it by doing things like that.