Obviously, the rule allows too much subjectivety. But since we can't re-write it today, I say a penalty should have been issued. One could go as far as to say Player A should have called it on himself.
Intention needs to be removed as much as possible from the rules, as it becomes too subjective. I've both benefitted and been burned by it in my sports career, as well.
I see it as black and white. It either hit or it did not hit the bag/chair/equipment/whatever. One could also say that the bag shouldn't have been placed there, since a roll-away was possible. It's unfortunate, but it's also the integrity of an individual sport where there are no referees.
On a similar note, a two-stroke penalty happened yesterday in the PGA Championship. A player placed his club on the ground, behind the ball, in a hazard, which was a bunker. You may not do this in a hazard. You may, of course, normally. Apparently, this bunker was far off the faiway, behing the ropes. He didn't even know it was a bunker, yet he didn't contest the decision. The rule is a two-stroke penalty for such a violation. Knocked him out of a playoff and I'm sure reduced hiw prize money.