It's mostly futureproofing and QA.This one is interesting. I did not know this and allowed it. I think most of the time the cleric would attack and then bonus action to healing word, but I was allowing it to be cast as the action if they wanted.
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Bonus Actions were specifically created to let people do some special task once per turn without spending an action. Then BA stuff were designed with this in mind.
This is different from 4e were minor actions can be traded up with a standard action. Minor action were always designed to be worse than an equal cost standard action.
Letting people use a bonus action aspect as an action lets them do 2 bonus actions a turn. There are likely some BA stuff that is too powerful or shenanigany to be allowed to be done twice a turn or combined with another BA.