Yes, I agree, it's all because of CBT. Plus, I would say, the insistence on self-assessment. Many of these impossible-to-be-sure-what-they-want rules -- certainly the ones that affect me -- could easily be settled with a relatively simple IT system, leaving self-assessment for the exceptional cases or those who prefer it.
As for the UK-Belgium treaty, it seems that has its own mysterious ambiguities. In the course of trying to find out what I had best do in my own case, I stumbled across https://books.google.co.uk/books?id=cRHVlebhViUC&pg=PA382&lpg=PA382&dq=us-uk+tax+treaty+%22public+funds%22&source=bl&ots=peCNO24SMX&sig=pZn4tKckJP747vJPj__dqN_jcQU&hl=en&sa=X&ved=0CDYQ6AEwBGoVChMI-9WnuYPgyAIVQjcUCh20nADN#v=onepage&q=us-uk%20tax%20treaty%20%22public%20funds%22&f=false
It's not actually relevant for my own case, being about pay rather than pensions, but it shows that the vagueness and confusion aren't unique to the UK/US Treaty.
It all does rather raise the question of basic principles of equitable administration of the law. Assuming the aim of the Double Taxation Treaty is the avoidance of double taxation, I would say the Treaty writers probably didn't set out to double-tax a UK citizen on two separate UK-public-funded pensions, which double-taxation (the said pensions representing virtually the entire income of said UK citizen) could only have the result that the UK citizen would be obliged to seek assistance from UK public funds... Talk about capital flight