These uniform rules are bullshit. Uniforms get lost and worn out. Eventually, the number of uniforms a school owns is less than the number of athletes on its team. When this happens, they have to order more. Manufacturers often change the designs they offer, or schools change vendors. So now instead of buying 5 new uniforms to resupply, you have to buy 30 or 40 or 50 to keep in accordance with these rules. Many schools don't have the money to do that, or don't want to spend the money to do that (and don't see the importance--it's rarely the coach doing the ordering).
Also, any coach who feels it's important to DQ runners from a direct competitor is missing the point. Yeah yeah, "rules are rules." But what kind of message does that send to your own runners? "Sorry guys, I don't think you're good enough to win on your own" or "Hey kids, the best way to beat someone who you've been training all year to beat is by taking advantage of a technicality." How bout you just beat them in the race?