The primary difference is there are 10-20 times more lower order cabinets compared to higher order. Also there is no expensive equipment within the lower order cabinets. If one is vandalised it may affect 10-20 customers at a time and to be honest they aren't vandalised that often. You have to look at it from a company point of view and the return on investment to install alarms, monitor them and react just isn't there. The cost of repairing the odd damaged/vandalised DP is minute compared to actively monitoring and closing the doors.
To answer your other couple of issues, very few customers actually demand compensation and when they do they recieve peanuts, so this really isn't an issue. Secondly I agree there is the possibilty of customer fall out from disrupted service but I believe ntl would regard this as acceptable losses.
From personal experience I would say that ntl are far more intersted in gaining new customers than worrying about it's current customer base, mainly because the majority of people are inherently lazy and will not jump ship, so why should they worry?
There is an obvious financial undertone to all that ntl does, you have to remember that this a company run by the banks to make as much money for it's shareholder as possible, full stop. Everything else, including you and me, is an after thought. Once you accept this life becomes much easier!!!