Its illegal to collect weed seeds? That's a first for me. That's a law I'd feel comfortable ignoring. I'd love to have a conversation with whatever law enforcement personnel was going to write me a ticket for that. "Officer, removing the seeds from non-native invasive weeds is bad how?" They should thank you, not punish you...
Also, I'm totally comfortable using seeds from areas sprayed with weed killers. I wouldn't feed any weeds from those areas, but the seeds are fine. Most weeds killers degrade fairly quickly anyway, and because you are growing from a seed, none of the new plant that is growing will have any residual weed spray on it. I collect seeds from areas where weed killer is sprayed every year.