Well, from what you go on to say, you seem to be agreeing that it is not currently the case, so there's obviously no need for them to 'mention it'. If it ever becomes the case that accepting a 'smart' meter does preclude entry into a fixed-charge contract then, unless/until smart meters become mandatory', I would expect the regulator to require the suppliers to make it clear to customers that this would be the consequence of 'voluntarily accepting' a smart meter.
Having said that, should such a situation arise, I don't think that action by either the regulator or supply companies would be required. There are far more than enough 'anti-smart-meter' crusaders out there that, should it happen, they would be shouting the message to the world from the rooftops, even if the supply companies' lips remained sealed and the 'small print' of their contracts remained in Fort Knox!