I'm pretty sure this is not a "political" decision, but just a recommendation their lawyers suggested to avoid being dragged into the (unavoidable) litigations that will spring up now that people wake up and realize they have been scammed. Google doesn't want to be nowhere near when the shit hits the fan.
It would be one thing if they followed a strict rule such as "we will block ads that present unrealistic claims" and then applied that rule equally across all advertisers. But the concern is they are singleing out one particular technology. That will harm those cryptocurrency business which aren't making claims about certain percentage of returns, thereby making it hard for a potential new novel use of cryptocurrency from being heard.
That does indeed seem to be what they're doing. For example, they're intentionally still allowing ads for Contracts for Difference, rolling spot forex, and financial spread betting (with some restrictions). Those are sophisticated financial products that probably shouldn't be used by normal consumers, and they're being marketed to that group via YouTube and Google ads by the exact same companies that have been marketing cryptocurrency trading, often via ads that bury the fact they are CfDs in the small print.
novel uses of cryptocurrency will almost inevitably gain traction because of their worth, especially considering how hot the market is right now with whales willing to throw ETH/BTC at anything that has a tiny chance to moon. how would Google enforce a rule like "we will block ads that present unrealistic claims?" who defines unrealistic? if you left that definition up to me 99% of the ERC-20 token based startups out there are unrealistic. there's no way that Google could "apply that rule equally across all advertiser" without being accused of being impartial/having vested interests.