I have to slightly disagree with the article. In the state of Florida, there is a law prohibiting private homeowners to grow their own oranges. This is to prevent diseases and fungus to spread to the orchards of the 'professionals'. Since the average homeowner isn't able to properly protect their trees, they're forbidden to grow them.
To compare this to magic, if 'amateurs' wouldn't affect the 'professionals' at all, then there wouldn't be a problem. But like was mentioned earlier, this isn't always the case. Often, people aren't exposed to 'real' magic. Their entire understanding of our art is biased because they've only seen bad performances, thus making them less reluctant to see a good performance (after all, they can't know before they see it that it'll be good). If the 'amateur' gives a bad performance, it may ruin the spectators outlook on magic for the rest of their lives.
However, if an amateur would respect the art and give magic a good name, then I don't think it would harm professionals. On the contrary, it might even help, since magic as a whole becomes more respected. And don't forget, just about everyone has to be an amateur at some point