Web frameworks consist of a lot of things besides pure programming. And these beginners barely understand programming...hell, they barely understand basic file systems that don't involve folder icons. And so all of this stuff...databases, MVC design, object-oriented concepts, dev ops, debugging...oh, and the entire world of front-end design...is thrown at them in such a way that there is no way they can decouple what are mostly orthogonal concepts.
It'd be fine if these beginners couldn't write a optimized SQL query in place of ActiveRecord...that'd be perfectly fine to work with. But I'm talking about a much deeper level of confusion. I once worked with a Rails self-learner and he wanted to throw up a single web page, with a sortable table that had a trivial number of rows (i.e. the data could all fit on one page without being cumbersome)...he could not, for the life of him, figure out how to do that in such a way that didn't involve creating a new Rails project and deploying it to Heroku.
It's not that I'm against teaching frameworks...it's just that for beginners, it seems like a very convoluted way to do so, and one that could be very unproductive in the end.