Except in this case, it is directly relevant to the legal issue at hand. When deciding a free speach case, the first part of the analysis is if the restriction is content neutral or not.
A content neutral rule is held to the standard of intermintent scrutiny, and is frequently upheld. A content based rule is held to the standard of strict scrutiny and almost always struck drown.
If the rule against signs on the overpass were enforced uniformly, then the white supremesists would not have a legal leg to stand on. But, at least based on the article, the rule is not being enforced uniformly at all; and is only being brought up now due to the content of the speech. That puts it squarly in the realm of strict scrutiny; giving the government a very uphill battle in court.