I doubt this is true.
see here for laws regulating scanners. In any case it's not illegal in California.
It is illegal to use a scanner in furtherance of a crime in California. And that gives law enforcement, again, an excuse to be suspicious.
Also, interfering with a train crew *is* illegal in California, and in this case, the police were called. So this guy's scanner use was arguably illegal in this case, as he was potentially using it in furtherance of that crime - it would be up to the police if they wanted to try to prosecute that (provided that initial call had not been canceled). Might not be very easy to make that charge stick, as it's not really the spirit of the California scanner law, but they could try depending on whether or not they wanted to be jerks about it. I wouldn't rely on the warm-heartedness of a cop, though.
The point is not that you shouldn't use a scanner because it's illegal - that's not what I said. The point is, and this is what I said up there, that it's
in your own interests to be discrete about it, and not give anyone a reason to want you to stop using your scanner.
Also, the guy was traveling, and just generally when traveling, it's also in your interests to know that scanner laws in some states can be more restrictive than others. For example, in New York it's arguably illegal to own a scanner without a permit (hasn't really been tested in court; the law itself is vague on what this applies to). It's definitely illegal to use a scanner while in a car; that's explicit. So it's best to use something of a "lowest common denominator" mentality when using a scanner while traveling, which means not drawing attention to yourself.