Court siding is when you take a device into match where it's not permitted, usually there is no crowd or anything, to relay what's happening to someone else.  You'll see in one case the guy had a device sewn into his shorts. 

If these players have someone at the match illegally relaying information to them, then yes, they are cheating.  Although it still wouldn't make 1xbit entitled to help themselves to the players balance. 

As I've explained already, I think what's happening is 1xbit not updating their odds fast enough. 

By the way, would you feel comfortable betting live Tennis on 1xbit, knowing that there's a possibility that you inadvertently place a wager on a line that 1xbit left open too long, which could result in your entire balance being seized?