As someone who works in the payments industry, there's some misunderstandings and misconceptions in this thread that are worth addressing.
These banks aren't "threatened", trying to "keep your money" for themselves or stop you getting better interest rates. If that was true they wouldn't let you deposit into investment platforms etc.
The fact of the matter is that Crypto is a largely unregulated market in the UK and there are legitimate fears that these exchanges can be used for criminal activity like money laundering and terrorism financing. When they say "fraud" they are referring to fraudsters stealing card details and using them to buy crypto which I can assure you happens often enough for banks to be concerned.
There's also the issue that customers can lose money either due to market volatility or scams and banks have a duty of care to their customers, but this won't be the main reason.
Of course for those of us who just use LB to get some weed on here that doesn't really matter to us, but our card issuers don't know that.
Having said all the above, I do think banks could relax the rules a bit or offer a feature to allow you to approve the transactions instead of flat out declining them. They could also be a bit more consistent in my opinion.
The good news is that Crypto has become mainstream so quickly that I'd be surprised if there wasn't some regulation coming which will address the issues people are having. I know that the Treasury were planning to debate crypto regulation in Q1 this year, but I'm not sure if they actually did that or what the outcome was.
