I agree on hit effects should apply when thrown, as long as the weapon has the "Thrown" property. My assumption has always been that if a weapon isn't a throwing one, then its on hit effects, along with its normal attack stats, don't apply. But I guess there could be an argument that they should. Hmm, I could be persuaded either way I think. Is this something 5e has a ruling on?
Indeed they dont't have the thrown property, but it's illogical. Imagine a melee spear whose magical thorns poison. Now you throw this spear and suddenly the thorns and / or the poison disappear or whatever happens just to not apply an effect... I'd like it better if they also used their effects as a throwing weapon and inflict.
Ok I just found out that there is probably a Feat http://dnd5e.wikidot.com/feat:thrown-arms-master for this, but as of now you can't test it.
But that should not be a reason for a lack of concrete description. At the very least, a better description should avoid confusion, and there simply isn't any.