Tax law. In the UK, every single payment in cryptocurrencies, however small, is a taxable disposal that you have to include in your tax return if your total proceeds or gains from all investments are above a certain threshold.
I'm not ideologically opposed to cryptocurrencies and neither is my wife. She's just allergic to anything that could potentially raise tax questions.
Edit: On second thought, I don't own a business, so I guess nobody is going to look into my tax fillings with the same suspicion since they do not expect me to be doing anything funny with my accounting.
Doing it correctly is non-trivial. You have to submit a so called computation for each individual disposal, which can easily run into several pages.
The algorithm for working out the cost of a disposal is actually a pretty interesting test case for learning a new programming language or paradigm. Try implementing UK share identification rules in SQL for instance :)
Separately, you've either misunderstood her position, or it's poorly thought out, and/or ideologically based.
What path would tax authorities use to ask Signal users (and only Signal users) if they've used cryptocurrency?