Nick Clegg, Vince Cable and the Lib Dems just don't get it. MPs' surgeries, supported by taxpayers' funds, are indeed confidential and are for the purpose of dealing with constituents facing some difficulty or other.
The confidentiality of a surgery does provide cover for the MP to parade their egos, to boast of their use of official powers to pursue a personal ideology or a secret party political aim (itself wholly reprehensible behaviour, well exposed here!), or to insult other MPs. The MP must know that, and should appraise the constituent, real or impostor, of the form, rather than themselves misbehave.
Too late to appeal to the third umpire; they were naive, and well caught behind the wicket.