If the spectator were to see the cards and make a decision it could presented as mindreading or psychological influence, but as the spectator doesn't see the choice and neither does the performer does it therefore fall into the more "magical" sphere of influence I wonder?
Totally agree. It can't be presented as mindreading, body language, muscle reading or even suggestion (although at a push suggestion could sort of work).
They way I have always presented this is as an experiment in the ability to predict the outcome of a random event. We already know that ESP cards are used for such Psychic testing experiments, and future prediction has always up there with everything else.
The only problem with this approach comes when they say something like
"well why dontcha just predict the lottery then magic boy"
Don't under estimate this one. It may be obvious to you, but I find it isn't to lay people. Especially if you have already been doing something with similarly backed ESP cards. For example, try doing it after jeff Pierces Telepa-Three effect. They have been handling the ESP cards themselves for 5 solid minutes.