It isn't just the qualification (no matter how irrelevant you think it is), it is the justification of the skills you develop on the course (SPAG, analytical skills etc). It's also another method of screening. I never thought it would be that important after University, but so many non-pedestrian jobs have high requirements for GCSEs, luckily for me I took them way too seriously and got good grades.
Just because it 'shouldn't' be a requirement doesn't make it unethical or unfair to ask for them. Usually if an employer is mildly concerned because you have a shocking GCSE English Literature grade, they'll probably interview you to see how articulate you are. Or just put you in the rejection pile because you don't fulfil one of their requirements.