The firmware contains third party licensed components, such as Trend Micro's DPI engine or the Tuxera filesystem drivers. Asus paid for a licence to use them on specific models. Running these on hardware that was not licensed for them is illegal.
No, Asus won't take you to court. But Trend Micro, Tuxera, Broadcom and who knows who else MIGHT file a complain to Asus about a breach of contract, who would then be forced to close down the firmware, essentially killing my project along the way.
This isn't just speculation. One of these (which I won't name) DID actually file a complain a few years ago about it. It did get close to forcing Asus to closing down things to a point that would have essentially killed my project.
We got lucky once. Let's not chance it again just because a few persons want to save fifty bucks here and there, at the expense of everyone else. Tim's policy is to forbid any discussion about it on these forums, and I've made my own stance clear about refusing to provide any support whatsoever for this kind of mod - especially in the light that Asus are taking active steps to prevent it. Ask people who saw their router be automatically reverted back to stock firmware following an update due to these protective measures.