How to do things with words

Clive Stafford Smith is an eminent attorney, writing here about ways of reducing the US prison population, which had reached 6.5 million by 2014.

According to the most recent analysis, 52 per cent of the federal prisoners in the US are locked up for drug offences.  Of those incarcerated for marijuana offences, 44 per cent had no prior criminal record. So legalising drugs would instantly cut the entire prison population by half. This is not an unreasonable proposition - the US Global Commission on Drugs recently called for the decriminalisation of use and possession of marijuana, and in 2016, three US states voted to decriminalise recreational marijuana.  (TLS, February 3 2017, “Prisoners of Conscience”)

This passage comprises four sentences. Sentences one and three belong together and are undoubtedly correct. They apply to all drugs, hard or soft. Sentences two and four apply to marijuana only. Somehow the two propositions have become joined, so that something many of us would be in favour of, or at least would review with sympathy – the decriminalisation of marijuana – has become part of a justification for decriminalising all drugs, which is a very different proposition. It is a fascinating example, by a lawyer, of  placing unrelated topics close together with the result that the reader unwittingly accepts the reasonableness of the whole.  Perhaps the trick works when used in court. Is there a name for such a rhetorical device?