My guess is that their reasons include
o fear that they'll lose control of their brand if too many people know they can have a say;
o lack of cooperation between marketing and IT;
o and perhaps pressure from lawyers who are nervous about new-fangled new media.
Digital Naiv oder Digital Native. Was ist der "richtige" Weg im und im Umgang mit dem Web 2.0 - und Puzzlesteine zu Essen, Fußball und mehr ...