Sometimes people make a mess of some sort where they can't reasonably clean up after themselves. When I was a kid (about 9 years old or something), I puked all over a Eurostar table just as the train arrived at its destination. (There was virtually no warning - as a child, I rarely felt any warning of the onset of such illnesses - a quarter of an hour before, I'd quite happily eaten lunch.)
In these cases, although it is highly unpleasant, one has to hope the train operator has a plan to deal with it, because you don't have the kit. The same applies for accidental drink spillages or even dropping something made of a fragile material, like a glass bottle.
Leaving or dropping normal litter does not fall into the same category. It causes needless mess, a health and safety risk, damage to the TOC's reputation, and untold problems from the "broken windows" effect - once one person trashes a train, everyone else thinks it's OK.
Most trains outside inner London have bins and I agree sometimes signage could be better, but they are not normally hard to find if you take a moment to look. Bins between train seats are usually the least used - they are also generally too small, and the slide-out bin bag compartment usually comes loose and flaps around in the aisle - so I don't especially like those. There is rarely any excuse for ignoring other types of bins.
