To be fair, in the majority of cases (at least the ones I've seen), the pre-requisite of a 2.1 is often coupled with a pre-requisite of a certain number of UCAS points from your top three A-levels, which is a proxy for screening by university. It is unlikely that the theoretical individual who got a 2.1 from london met will have met the UCAS recommendation, making your argument somewhat invalid.
Another point to remember is that your university degree is only one component of your application and what recruiters make their decisions on. If you are relying on your grades alone to get a job then you are likely to be unsuccessful anyway. Additionally, even with the UCAS points and 2.1 restrictions, the pool of potential applicants is still massive, and the pool of actual applicants is likely to significantly exceed their recruitment needs. They don't need to spend the additional time and money sifting through the applications of people with 2.2's in the hope they might possibly find a gem in the coal. And even if they do, they still need to answer the question of why this brilliant individual only got a 2.2, with the answer often being a negative factor that reduces their chances of being hired.
Getting a 2.1 should be the number one priority when it comes to university. If you neglect this priority in order to socialise, get involved in societies/sport/student politics etc., or for any other reason, then you are doing university wrong. It really shouldn't be a surprise to students who graduate with a 2.2 that this filters you out when applying to graduate schemes, those who are likely to be successful in what is currently an extremely competitive jobs market will be working towards this next step well before they graduate.