I brought my wife over on the 90 day fiancee visa well before there was a TV show about it, and mostly because it was the only way we could hang out. Thankfully we lucked out, and celebrated our 20th anniversary earlier this year.
But to the point: When you're dealing with DHS and immigration, it's made absolutely clear that if you screw anything up, you're kicked out of the country and it's going to be a ballache to get back in. It's not surprising that they were not included in the blanket pardon, and I think that's actually pretty reasonable, even though I also think that incarceration over marijuana possession is absurd and has been pretty devastating to the lives of millions of people and the communities they live in.
The amount of work you have to do to qualify for a visa, and to move between the different statuses (there are fun periods where your status is just... pending. When we went through this, there was nothing you have to show for this except for a generic looking letter from DHS. All your fancy documentation is sitting in a US Gov't inbox somewhere while they make sure you're not a terrorist) and, frankly, if you manage to get arrested for smoking weed while you're on a visa, you're kind of an idiot.
We didn't even travel outside the country until my wife got her full-fledged citizenship. And when she applied for that, they gave her a hard time over the speeding tickets she'd gotten while here.
Whether you agree with the concept or not, visitors to the country are told pretty clearly that the rules apply extra hard to them, and the consequences of not following very basic rules are going to be extra inconvenient. You get caught breaking the law, you risk losing your visa status and losing the ability to ever visit the country again.