Amateurs lack experience and training. They could very well have good ideas for stories, but not know how to execute them. This can lead to stories that have choppy or clunky narration, stiff dialogue, or excessive description. All of these things affect the reader's experience, and they do lessen the quality of the story, but that doesn't make the story itself bad.
Amateurs need to know what the next step is (or be informed that there even is a next step). Amateur writing is what I consider to be a step in the right direction, but not the end of the journey.
Bad writing, on the other hand, is a step in the wrong direction. It's when characters are inconsistent and plot holes abound. It's not that dialogue is clunky, but that it simply doesn't make sense. If a protagonist is afraid that a group of spies is tracking his every move and listening in on every word he says, it would be moronic for him to have a conversation with a stranger and explain all of that; he should be assuming that this person is also a spy and not to be trusted.
I like working with amateurs. In general, they want to be good writers and will soak up whatever knowledge they can. Bad writers, though, tend to be stuck in their ways and totally incorrigible; it's kind of what makes them bad writers.