Whether that message is in the file or not doesn't alter the license it is released under. Is there a copy of the GPL or other open license in the addon package? What license does the author(s) specify the addon is under? I have done a number of open source work / projects where the other scripters would use a similar line in the actual code, not realizing the language and intent of them...
Personally, I have no problems with commercial addons being listed in the addon directory, so long as they are marked as such, and authors are honest about it. The amount of work that goes into developing some projects is immense. It's only fair that people have the right to choose between commercial and non-commercial addons, so forbidding one or the other is akin to denying a person's right to free speach, as you try to "sweep them under the carpet". If you don't like the license something is released under, why not either write an addon / plugin yourself, or find one that does the same thing, but is released under a license you are comfortable with?
Even now, I am working on both open source and closed source addons and mods for WP. Just because some are released with a commercial license versus an open source license does that make them any less relevant, or useful to some? Why can't an addon for WP be commercial, and not open source? Trying to say that an addon can only be open source or closed source, takes away an author's or artist's god given right to choose how to license their creation. Censorship stinks... Ask that things be honestly marked? Yes... Tell people they can only develop for WP or list their work if they are open source? No... Don't need people trying to dictate to others what they can and can't do with their work. But that's just my two cents as an end user, scripter, graphics artist, and programmer who works in a number of languages on a number of different platforms.
*Gets down off of his unintended soap box...*
BTW: If you can read the line, and there is a GPL or similar license included to cover the addon, what would you have to reverse engineer? The code wouldn't be encrypted if you could read the notice in it in plain text...