My only ‘exposure’ to American-English is on CNN and I have only noticed this ‘shtr’ in the last two years or so.... it is indeed very strange and I cannot see any reason for it. I am also hearing more and more people using the hiatus R... as in ‘the ideaR of it’. Very definitely a very British thing, maybe even English more than British. It certainly is not a feature in Irish-English thankfully. It sounds so awkward and unnecessary.