I guess the concern from the park's perspective is that if someone has a camera out, they won't be holding on, which risks a person falling out of their seat / into the water. There's also the case where someone could drop their phone into the water and jump in to grab it. Unfortunately there's a lot of stupid people out there.
Towers do say on their map and website that filming and photography isn't allowed on any attraction (so technically not even the monorail or Skyride if we wished to be pedantic). To say it doesn't explicitly say it on a ride restriction board is a bit of a grotty loophole which ends up with us having stupid, spoon-fed restrictions and over the top safety rules.
This specific situation is obviously all the more high profile because we're looking at a well known enthusiast, who is an ex-employee of the park. It sets an example to over 20,000 people. Can understand why Towers don't particularly want them to go 'hey, we're breaking park rules and we get cool stuff from the park'.
There's also the cynic in me which wonders if this is Theme Park Worldwide trying to just get more publicity. All publicity is good publicity as the saying goes..