Since, what 526.6 "actually says" includes no exceptions for connections in particular circumstances, it appears to require that no connection be subjected to "appreciable mechanical strain". In a situation in which 'mechanical stress' will inevitably be applied to connections (e.g. when pushing connections to an accessory into a back box) , the only way of ensuring that such stress does not result in "appreciable strain" would be to employ some appropriate means of 'strain relief'. Since that is very rarely practical/possible with accessories, we don't do it, but that doesn't alter the fact that such an omission constitutes a contravention of what 526.6 "actually says/requires". Just as with "non-combustible CUs", a regulation should not require something which is 'impossible', but this one appears to do that.