I would wager in the case of De Niro and many others named it's a situation of them simply not wanting to see toys/etc made of their work and lining the shelves of shops, rather than a desire to avoid the hassle of approval or anything like that.
I of course like these figures, see nothing wrong with most of them, but the idea of making a toy from a film like 'Taxi Driver' would be taken as a joke by most people just as the idea of a Travis Bickle Beanie Baby surely hits you as a joke. Outside of those already buying these figures, I would say it's not unreasonable (and in some cases not wrong) that an actor would think that agreeing to such a thing would cheapen a film or their image or be in some way cheesy or what have you. They put all this care into a film, it really means something to them, and now one day someone wants to make an action figure out of it and the only thing that could be stopping them is laziness? After a HT Travis Bickle figure, what would be next? A Travis Bickle bobble head? Maybe after that a Travis Bickle in his taxi, pull it back and let it go and it speeds off?
I'm not saying such thoughts are right or wrong but just that I would wager such thoughts are what's really behind why you don't have an official Travis Bickle figure and some other actors people are asking for, that I doubt it's a matter of there being some actors who just "can't be bothered."
