I wrote something about this subject too and what I was told was the following:
The fact is that there are a lot of processes that are run on words when they are moved to known. It is just not that easy to undo. We do warn users that this happens initially and otherwise, it’s really not that big a problem. Just click on words you don’t know when you come across them and make LingQs. The process is the same whether they are blue or not.
It’s a technical limitation as of right now.
You’ve got to think about how complex their system is. LingQ has to track hundreds of thousands of words for some users, and every single one of those words is stored at different levels of understanding (known, learning, new, etc.). Building and maintaining a platform that can handle all that data for every user must be pretty complicated.