I could never see the point behind a blanket ban on all Crypto currency related Ads. Take a selective group of employees and identify the good services, from the bad services and only allow Crypto currency advertisements that are reputable and legitimate.
Yes, I agree some advertisements for "bad" services must be banned and it will even be beneficial for Bitcoin if it was banned. Also, a good customer review and reporting service will help to curb some of the scams that are doing the rounds.
I agree with you, much better approach would be to check all who apply for advertising - but it is obviously too expensive or too complicated even for such big company such as Google. On the other hand Google is check individually for their AdSense service, and they pay close attention to that question, so this means that such an approach would be possible also for AdWords service.
Since Google is only include USA&Japan in permitted countries I do not think that means too much globally for cryptocurrency, especially because it is only about exchanges. I also agree that exchanges like Binance or Coinbase do not need too much this type of advertising, they have good referral systems and their users promote them in the best possible way.
The conclusion is that Google and before them Facebook are looking for ways to get a part of crypto cake, which is quite normal in the business world.