While I understand your frustration with this and I agree that it could be somewhat more more clear, at least in the case of quest chains, I really do not think that it is that big of a deal.

If a quest cannot be completed due to level restriction, it is marked out as red and you cannot finish it. You can, as you stated, complete the necessary steps for the quest, but in your quest journal and with the npc that you started the quest with the icons are distinctly red, meaning that you cannot turn it it until you meet the requirements.

I am pretty sure that allowing the player to be able to accept a quest and not be able to turn it in has to do with the way the quest system is designed, essentially taking a quest and turning it in are two separate things so that the requirements put on the latter do not translate to the former.

My guess as to why they did this is that so people could start the quest and level up while doing the quest which would then put them at the level requirement to turn in the quest.

To me it has never been an issue and I have never had a problem discerning which quests I would be able to complete and turn in and which ones I couldn't based on level requirements.

Also, if you are able to complete the quests and then not turn them in you will just have a whole backlog of quests that you can then get the rewards for once you hit the required level.