Language A Levels taken by native speakers are considered to be less valid by universities as they are second language qualifications. They are not designed for native speakers. There is a small literary component in terms of reading a short literary text and writing an essay about it, but it is by and large a language exam designed for students acquiring the skills of speaking, reading and writing a second language they have learned ab initio rather than as a mother tongue. A quarter of the course (or thereabouts, I forget the exact % as haven’t taught A Level in a while) is examined through an oral exam - a child who speaks the language at home will quite effortlessly get full marks.
As such, if you grow up speaking, say Mandarin, or French, or Russian at home and can speak it fluently, then you are at a huge advantage over fellow students without that exposure. It does not present the same level of challenge or achievement for a native speaker to pass a second language exam compared to a student whose only exposure is learning it at school. I did French A Level many moons ago and it was incredibly hard as someone with no exposure beyond the classroom - it required hours and hours of study at home. A native speaker would not need to put in hardly any work in comparison. This is all understood by universities and as such, any school worth its salt will not allow a native speaker to take a second language A Level in their native language as one of their three main A Levels.
Of course there are degrees of fluency and exposure of native languages at home, and I have certainly taught many bilingual children who couldn’t read or write with fluency in their mother tongue due to never being schooled in those skills. If a child were in this category, then a clear explanation of this to the university would be required if the child took their ‘native’ language as a third A Level. A child with a very obvious surname connected to the country the language they are taking an A Level in should always have an explanatory note attached to their reference to explain the level of fluency as universities may
make assumptions if they see, say, a child with an obviously Russian name taking a Russian A Level.
All this is nothing to do with the perceived usefulness of the language being studied. That’s not the issue. The issue is fairness for all candidates. A native speaker of a language is not starting from the same place as a second language speaker and allowing them to take the same qualification and according it the same value is not fair. Plus, if you have a large cohort of first language speakers taking a second language A Level, this increases the grade boundaries and makes it more challenging for students who are studying the A Level as a second language to get higher grades. It would be like an English child moving to a French school in France and taking the French Bac with second language English as one of their ‘specialisms’ - of course they’d get full marks. How would that be fair on the other students in their class? How could a university judge this students’ abilities in the same way as others in their cohort who didn’t have the same linguistic advantage?
This is why native speakers taking qualifications designed for second language speakers are so problematic. Hope that clears up the confusion. I am an international school teacher so have lots of experience with just this issue!