The DMG points out clearly that you are not supposed to include items that no one can use. As soon as you are getting away from the way you are assumed to award treasure it has unintended consequences. This is one of them. It isn't a huge deal really, but if you use a rules as written argument for why they cannot use an implement you've screwed up as you aren't using loot as written.
I thought really hard about it but I couldn't find a reason to punish Clerics and Paladins. Giving them the extra hurdle of having to either make their own implements or somehow find the correct ones didn't accomplish anything. They have no more character power than Wizards and Warlocks or classes that just care about magic weapons.
For the most part a generic, out of the PHB symbol, will mold itself to the spiritual inclination of the user. There will be exceptions of course but for now a focus is a focus is a focus.