Well, following that statement to its logical conclusion, any laws would be, by their nature, as fundamentally flawed as gun laws, so why bother passing them at all? I'm not saying that recent gun legislation hasn't been deeply ridiculous, and I'm not saying that better enforcement of existing gun laws wouldn't be far more effective, but "laws only affect the lawful" is a pretty weak argument against legislation.
Here's the important difference, and I think you understand where I'm going with this. Creating a set of restrictions on an object, like when/how/how many/what kinds of guns you can purchase legally does nothing to stop the criminal (or potential criminal) since said criminal does not acquire his gun in a legally sanctioned way. I'm not even going to go down the "But we're depriving citizens of their ability to defend themselves" road. It is flawed thinking when lawmakers believe that these restrictions will stop those who use guns in the commission of crimes. Your "logical conclusion" argument is weak for the following reason: A law should have a purpose and realistic aim. Murder, for example, is already a crime and those who kill are prosecuted. The prohibition against it probably deters some people from killing, but it is fundamentally a tool to punish perpetrators after the fact and that is a good thing. Gun laws are in a completely different category and do not limit the crimes they seek to redress either before, or after the fact.
Brett