The subject grades for A-level are often specified, for example in the Cambridge A-star AA standard offer - it’s at the admissions tutor’s discretion and it’s often the case that specific grades are required in particular subjects, depending on the degree course. And it’s very subject-dependant. Some degree courses require a certain subject content or skill threshold at admission, or students won’t manage the course (medicine, STEM, maths, modern languages, some humanities). Others don’t require this or are studied ab initio (law, social sciences, philosophy, etc.)
If you applied to Oxford to do a STEM subject with only standard rather than higher level sciences and maths at IB, you wouldn’t be equipped to do the course because you wouldn’t have covered enough of the essential subject content and mathematical skills needed before arriving. I’m not sure why this is surprising, though?
On the IB/EB/French bacc and their system: well, the French and U.K. university systems are completely different. The French system is very hierarchical, with high drop-out rates at the public universities. Oxbridge isn’t quite comparable: it’s more like one of the grands ecoles. A student wouldn’t expect, for example, to go from the bac direct to a course at one of the grands ecoles, for example: they’d typically be studying at another university or a preparatory course for another two to three years for the competitive concours exams just to get in for, say, engineering.
Whereas you could apply direct from an IB to a four year engineering degree at Cambridge; but for that, though, they would specify particular grades in your IB higher level subjects, because you would already need a threshold subject requirement to cope with the degree. Not sure why this is controversial. Each HE system is different, and in many European systems students are often locked out of specific academic strands or professional qualification routes and courses at much earlier stages of their educational career, UNLESS they are willing to transfer across to another strand, and do several more years of preparation.
It’s different again for US degrees, where it’s also very common to split a degree between two institutions and do, say, two years at a community college in order to get up to the requirements for a further 2 or 3 years at a more prestigious college; whereas U.K. universities tend not to favour transfers. Likewise, there are subjects available at undergraduate level in the U.K. that are are generally graduate courses only in the US (eg. medicine, law). Each system reflects the way the transition from school to HE has evolved, and it’s not especially difficult to transfer between systems at 18, but matching parity of qualifications also has to recognise that there are differences as well.