The tube is private property, it's their rules!!!
FACT: YOU HAVE ALWAYS NEEDED A PERMIT TO TAKE PHOTOS ON THE TUBE!!!
Sad I know but I used to snap tube architecture and trains for a hobby, I stopped about 8 years ago, it was just too much trouble!
At that time you simply had to apply for a permit to take photos on the system, I believe that has always been the case, you never had to pay for it but the tube is private property and lots of people don't know that. So you write for a permit letter, free of charge, then present it to the Station Supervisor at each station you wish to photograph at. You collect a visitors pass and you get on with it. At the time I was doing it, it was mainly for fire regs, so if they had to evacuate, they knew you were on the premises for business other than travel.
I was always careful to never catch kids in my pictures, even 10 years ago, and where possible I would always avoid adults as well, mainly for clear shots but also to avoid and trouble with official jobsworths and nutters. And I can tell you there are lots of jobsworths working down the tube, and plenty of little Hitlers who have worked their way up to station super, just want to be difficult, most were really good and genuinely interested, but some were utter twats.