In Some States now the seeds are legal but still sending seeds into the US is illegal. This has to do with federal law. And it is not just Cannabis seeds, it is about plant material, that it can be potentially harmful because it can spread virusses and exotic germs.
BUT most of the UK and Dutch based seedbanks send it anyway
And they use all kind of techniques to get it to you on time. I don't know if you can be arrested for this, they will just destroy the seeds.
I recently ordered here http://ilovegrowingmarijuana.com took some time to arrive but the seeds are quality and all 10 seeds sprouted which is a 100% germination.