Unionisation of freelancers is always very, very difficult to arrange. I'm sure a lot of photographers don't even realise the NUJ is relevant to them. And freelancers by nature are solitary. It's not like we'll be standing in picket lines, yelling 'scab' at people who are taking phone calls from editors.
I'm not sure what the solution is. (Actually I have an idea, but I'm still wondering how to implement it.)
But it is an issue as to how unions can support freelancers, for all the reasons you mention and more. Ad astra per aspera
