Conversely, they could be setting up different weapons having different crit values, but wanted to test a more generic version first. What if the next iteration of the rule stated "When you crit with a weapon or unarmed strike that you are proficient with, in addition to the normal weapon damage, you also roll the Crit dice of that weapon and add it to the total." And then you get things like
Dagger (crit 1d6)
Club (crit 2d4)
Short Bow (crit 1d4)
Long Bow (crit 1d6)
Rapier (crit 1d4)
Long Sword (crit 1d8)
Great Sword (crit 2d6)
Great Axe (Crit 3d6))
This would further differentiate weapons, is easily marked on a character sheet, and could even be included with monster weapon attacks
Add on to that monsters recharging on a crit (which also encourages DMs to always use recharge abilities first), and suddenly you've got a much more interesting crit system than before, while not being too complicated. Players just need to look at their weapon damage to see what they roll when they crit. DMs just need to mark the recharge for their monster)