Why not?
You can't just go around making arbitrary, poorly thought out statements like this and expect anyone with a functioning brain to just accept it because you said so (and honestly, it's got to the point where because
you said so is actually becoming a reason to find a statement dubious
).
Harder content should reward better gear because reward should generally tied to effort/difficulty. This is the nature of progression raiding. However the flaw in your thinking is that because it's possible to get a random lucky drop from LFR, that means that LFR rewards better gear. That is easily proven false by considering the following scenario:
Take 100 raiders doing every LFR boss every week for a month. Take another 100 raiders doing every Mythic boss every week for a month. Then compare to see who has the better gear. If you want, you can go and look through a bunch of armory pages, but really, that is unnecessary since anyone can tell you the result.
It's not even going to be close. People who raid Mythic are going to have significantly better gear than not only those who raid LFR, but even above those who raid 1 level lower at heroic.
So, allowing the odd TF item to drop from a format like LFR doesn't break anything. What TF does achieve is to allow for people doing that content to gradually upgrade their gear, thus providing a reward mechanism for commitment and effort (which most people would agree is a good principle to follow).