It boils down to the intention. The intention is to download videos to your computer (as viewable mp4 files for end users) which isn't legal, as they are intended to be streamed (or possibly removed in the future) with ads, stats, analytics, recommended etc.
I definitely don't defend this and I know that this whole copyright and DRM thing is fundamentally flawed by design, but it unfortunately works this way legally.
If it boils down to user intention then you can't also argue the tool is circumventing anything. Either mimicking a browser and working the way google expects any client not written or authorized by them, such as a web browser, to is ok or it's not.
Also, I asked what is being circumvented, not what the tool does.