It's not an enforcement issue so much as it is a heavily exploited loophole. Part of the reason freight trains are so long is so that they can't fit in passing sidings. Since Amtrak does fit, they end up having to yield because the freight trains simply cannot.
Could this be fixed by legislation on max train length to ensure all trains fit in sidings? Yes. Will that legislation get passed? No.
Could this be fixed by legislation on max train length to ensure all trains fit in sidings? Yes. Will that legislation get passed? No.
An interesting video on the subject: https://youtu.be/qQTjLWIHN74?si=t3u3iyZj1kRQQUCe