Is this fine even legal?

Here's an excerpt of an article from the Wall Street Journal from June, 2012 that raises the question "Is that even legal"? How does one have the right to fine their employees? Especially for such a dumb thing like this.

Excerpt:
"I cringe every time I hear people misuse 'is' for 'are,' Mr. Silver says. The company's chief operations officer, Mr. Silver also hammers interns to stop peppering sentences with 'like.' For years, he imposed a 25-cent fine on new hires for each offense. 'I am losing the battle,' he says."


Source: http://online.wsj.com/article/SB10001424052702303410404577466662919275448.html