I agree though I think the general reason behind this is to not punish first-time players who at some point in the game find a new cool weapon that they want for a character and feel they can't because they haven't invested a proficiency point into it. This is something of a tricky point, because regardless of the game making it very clear that proficiency only grants a modal and has no negative side-effects if you don't take it, the fact that it's a spiritual successor for a series of games that heavily punish the player for using weapons they're not proficient at means a lot of players come with the preconception or psychological reflex of assuming one *must* use weapons they're proficient in and *must* invest proficiency points based on the weapons they intend to use and not the modals they grant.