I support people being drug tested to receive welfare. I think having random drug tests while being a foster parent is a good suggestion. I do not think that background checks to look at criminal records is enough. Agencies should look deeper into the family to ensure a positive environment.
Drugs alone don't cause people to be abusive, what people do in their private home is no business of yours, what matters is if the kid is safe which in 99% of circumstances they actually are because foster parents volunteer.
Honestly making foster parents jump through more hoops then they already need to is a bad idea, especially with the massive lack of fosters parents the US has currently anyway and with 1000s of kids growing up being wards of the state means they will never have a normal life.
If the worst thing a foster kid sees is their foster dad smoking some marijuana (and not hurting the foster child) then I would say the kid is luckier then the 1000s of others that live in orphanages until they are old enough to leave.
No history of drug abusing people adopting kids, and even if, how exactly are the kids being abused just by having a parent that does drugs? Unless the kids are seeing their parents using drugs, or witnessing their parents being strung out it isn't child abuse. Not everyone is entitled to a perfect life with perfect parents in a perfect house. Just more requirements for good parents to be denied so that the real child abusers, who typically abstain from drugs, can adopt them.