Rhetorics and narrative to prepare the public for a cap on student visas

So the government announced it was going to extend its cap to the student route. How and to what extend is yet not clear, but what is sure is that they rolled out the perfect narrative to prepare the British public to accept a move which could very severely damage a vital sector for its society and economy.
The report on which Damian Green's speech was based is itself rather neutral, but the tone of the minister, or that of the media which reported on it was far from that. The Guardian made it sound as if swarms of students stayed behind illegally in the country, when all they do is switch over to another tier, with the permission of the UKBA, because they have been made job offers. Sadly, the BBC is hardly any better. According to the Daily Mail, it is students undertaking studies at lower than degree level which might bare the burden of this protectionist mania. According to the paper, they might be barred altogether from coming into the country. To say that they are not as valuable as university foreign students is to either show contempt for studies below degree level, or to ignore how vital for the economy this educational sector is.

One major question remains. On what basis did the government decide on the figure of 100.000 of migrants per year? We have so far been given no justification why this number is just. Or is it just one of those political and psychologically magical numbers?