The PCN offers you an opportunity to accept liability and pay the penalty immediately, and in exchange for that you get a 50% discount. You are entirely within your rights make representations and if those are rejected, you have a legal right to go to the tribunal and at that stage, TFL will have to disclose all the evidence it relies on. The problem is that by then, the discount will be long gone.
This is no different to a criminal case where someone might be offered a fixed penalty or a police caution, in either scenario the police don't need to disclose any evidence as the accussed has to be willing to accept the out of court disposal. The accussed is 100% entitled to go to court, plead not guilty and require the police to prove their case with evidence, but at that point the out-of-court option is gone. Even if no out-of-court disposal is ever offered, you get up to a 33% discount on any criminal fine by pleading guilty at the earliest opportunity (which again saves the police and the CPS the trouble of proving their case at trial).
There is a compelling argument that if someone is willing to accept liability without putting the state to the trouble and expense of proving its case, the accused should be rewarded with a discount on the punishment.
There is a counter-argument that such arrangements will incentivise some innocent people to accept guilt or liability to avoid the risk of a more severe punishment.
Whether these arrangements should be the way they are is a political issue rather than a legal one of course, when dealing with a live case we can only deal with the law as it currently stands.
All that being said, TFL do voluntarily provide the video, so it's all a bit academic.
