I think they're asking how regulating sales will prevent straw purchases when straw purchases are already illegal, so presumably additional regulation won't do anything more. If you're convicted of a straw purchase you won't be able to legally purchase anymore.
And also the vast majority of straw purchases and lying on background check forms are not prosecuted in the first place, despite there being clear evidence for a slam dunk case. Hell, if you're the president's son you can lie on a 4473, have your girlfriend steal the gun and throw it in a dumpster, and then write a book about it and you won't get prosecuted.
Zomg, I typed that info into duckduckgo and found this: