Marijuana is still illegal according to the world anti-doping association (WADA), This band isn't coming from the US anti-doping association or USA track and field. It doesn't matter if a band substance is legal in your country, marijuana is still illegal in over 100 countries around the world.
You can't have a worldwide doping association that says "well XYZ substance is legal in this country, so it's fine if those athletes from that country take it." They have established rules that all Olympic athletes must follow.
I'd imagine a lot of athletes smoke marijuana or do recreational drugs, the key is just not getting caught and not getting caught where a ban will force you to miss a competition.
Richardson chose incredibly bad timing. A 30-day band isn't that bad, it just so happens that within the next 30 days she'll miss the biggest professional event of her life something she trained years for. It's unfortunate.
Maybe it's because I was in the military and had to deal with random drug tests for several years but, I don't feel "sorry" for her at all. She knew the rules and she chose to risk her career at the least opportune time possible.