As 04nbod says, "No one expects a history lesson but a polite sentence." (I'm not sure how much I agree with an expectation of politeness, but other than that) To apply your metaphor, it is the difference between saying "Ow, dammit, you stood on my foot." versus just "Ow, dammit." and storming off angrily leaving the foot-stepper to wonder what just happened (was I gripping their hand too tight? did someone bump into them? do they have a bad back that makes that dance move uncomfortable? did they step on a nail?)
No one should be expected to teach a seminar on race/gender/etc relations every time someone offends them, but it is certainly more useful for all concerned to give the offending party some idea what they've done. Though I do agree that it is not the burden of the offended to explain anything if they don't want to, 04nbod isn't wrong that a little explanation can go a long way among well-meaning people. "You stood on my foot" (or rather the applicable equivalent) is plenty.