This is an easy question once you have been clear about what "human behaviour" is. According to enwiki, it is a "range of behaviours *exhibited by* humans".

What would anybody do with this data? In what application could it be of interest?

Moreover, as a great Icelandic ontologist once said: "There is definitely, definitely, definitely no logic, to human behaviour" ;-)

I think "constraints" are already understood in this way. The name comes from databases, where a "constraint violation" is indeed a rather hard error. On the other hand, ironically, constraints (as a technical term) are often considered to be a softer form of modelling than (onto)logical axioms: a constraint can be violated while a logical axiom (as the name suggests) is always true -- if it is not backed by the given data, new data will be inferred. So as a technical term, "constraint" is quite appropriate for the mechanism we have, although it may not be the best term to clarify the intention. 

Some important ideas like classification (instance of/subclass of) belong completely to the analytical realm. We don't observe classes, we define them. A planet is what we call a "planet", and this can change even if the actual lumps in space are pretty much the same.

Now inferences are slightly different. If we know that X implies Y, then if "A says X" we can infer that (implicitly) "A says Y". That is a logical relationship (or rule) on the level of what is claimed, rather than on the level of statements. Note that we still need to have a way to find out that "X implies Y", which is a content-level claim that should have its own reference somewhere. We mainly use inference in this sense with "subclass of" in reasonator or when checking constraints. In this case, the implications are encoded as subclass-of statements ("If X is a piano, then X is an instrument"). This allows us to have references on the implications.

In general, an interesting question here is what the status of "subclass of" really is. Do we gather this information from external sources (surely there must be a book that tells us that pianos are instruments) or do we as a community define this for Wikidata (surely, the overall hierarchy we get is hardly the "universal class hierarchy of the world" but a very specific classification that is different from other classifications that may exist elsewhere)? Best not to think about it too much and to gather sources whenever we have them ;-)

At the moment, hard constraints (from definitions) and soft constraints (expectations) are simply mixed, and maybe this is fine since we handle them in a similar fashion (humans need to look how to fix the situation). Most constraints, even those that refer to definitions, are rather soft anyway since we apply them to statements, not to hard facts. Hard constraints can only occur in cases where the *encoding* of a statement in Wikidata is wrong (not the intended statement as such, but how it was translated to data).