Why are people allowed to write laws when they can't accurately express their intentions?

Reading about the legal challenge to the laws surrounding prostitution, it seems apparent that the intention of the legislation was to prevent people from being forced/coerced/exploited into prostitution, without getting people in trouble for being prostitutes.

We can all wrap our brains around that concept. Even if we don't agree with it, we can easily grok what's being described. Even if you believe that all prostitutes are being forced/coerced/exploited - even if it turns out that in reality every single prostitute in the world is being forced/coerced/exploited - we can still conceptualize the theoretical difference between being forced/coerced/exploited into prostitution and willingly engaging in prostitution. It isn't a particularly difficult concept. (Just like how we can conceptualize the difference between a horse and a unicorn even if we don't believe in unicorns.)

So why can't they write the law to reflect these intentions, rather than making fussy and arbitrary rules about living off the avails and bawdy houses?

This happens quite frequently. They raised the age of consent to 16 in an alleged attempt to stop the sexual exploitation of minors, rather than writing legislation against the sexual exploitation of minors. I recently heard of a school that required students wearing uniform kilts to wear hosiery underneath to stop students from wearing the skirts too short and flashing their thongs - rather than just making a rule against flashing their thongs. They recently made age-specific changes to Ontario driver's licences to stop people from driving around with cars full of drunken screaming idiots, rather than making rules against driving around with cars full of drunken screaming idiots.

We can all conceptualize the nuance of the specific behaviour that these rules are trying to stop, and we can all see how the legislation as written doesn't precisely reflect the intentions, it just sort of generally correlates most of the time.

So why don't we demand competence from our legislators? Why are we, as a society, allowing people to write legislation when they can't clearly articulate their intentions?