In a long tradition of Western use of stickers as a medium for conveying messages to passerby, especially political ones, this driver references a then-commonly known phrase used to justify pride in the South using Confederate flag motifs. As the Confederates were negatively seen as hateful to minorities, people nostalgic for the Confederacy commonly used the phrase "Heritage, Not Hate" to justify said nostalgia.
In an interesting twist, this driver justifies his own pride in being American, which in their age of cynicism was seen as idealistic or even insane. It is unknown whether the driver noted the common thread of xenophobia between the two bumper stickers, but the analogy fleshes out well.