If true they are probably trying to protect him and/or his Israeli partner.[1]
Salem is reputed to be Mubarak's bag man. Allegedly, those "extra-legal" contraband deals were handled by the man in whom the former Egyptian dictator placed a good deal of trust.[2]
There have long been whispers that during the 1973 war with Israel, Egypt was betrayed from within. Salem is associated with those elements, rightly or wrongly suspected of the collusion, which rose to prominence with Anwar Sadat. I cannot speak to any of this, but his long and deep ties with Israel have done nothing to allay these rumors.
Whether he's being held (assuming it's true) to protect him or to keep him from talking is open to debate. Here is the loquacious Egyptian blogger Zeinobia's post on the subject:
http://egyptianchronicles.blogspot.com/2011/05/need-confirmation-hussein-salem.html?utm_source=feedburner&utm_medium=email&utm_campaign=Feed%3A+EgyptianChronicles+%28Egyptian+chronicles%29&utm_content=Yahoo!+Mail
[1]http://www.globes.co.il/serveen/globes/docview.asp?did=1000619367&fid=1725
[2]http://egyptianchronicles.blogspot.com/2011/02/mubaraks-entourage-man-called-hussein.html
