Frankly you're a lot of an idiot if you over-take a vehicle that is signalling right, as you're bound to disappear in their blind spot: whenever I see that happening, I either wait or pass them on the inside.
I'm not saying that filtering past a signalling vehicle is without risks - I wouldn't do it, for the most part, but the suggestion that the motorist can say 'well, that's not my fault' is not right. The fact that the blind spot exists is exactly why, prior to committing to a turn, the prudent driver does a shoulder check to ensure that the blind spot is
clear. It won't help the rider travelling at warp speed (but that's your contributing factor right there), but it will ensure that you don't nudge the fellah who's either not seen or misread your intentions.
A sensible rider will be up the handy gap the turning vehicle has created on the inside anyway
Filtering or overtaking slow traffic is the same, the balance on who was to 'blame' or commits an offence, if any, a driver has the ptential to say the m/c was in the blind spot and s didn;t see the approaching m/c. .
My earlier point still stands - there are some perfectly reasonable physiological explanations as to why a driver does not see a motorcyclist in some circumstances, but any argument that starts with "they were in my blind spot" is simple lack of observation. I was taught shoulder checks when I learnt to drive some fifteen years ago, I'm sure they're still taught today.
Yes, a prudent rider makes sure he doesn't find himself in conflict with other traffic, takes a pragmatic view and assumes everyone else is out to kill him. That doesn't mean other road users can abdicate their responsibilty to carry out simple observations prior to making a turn!