What they leave out
It's true that with a K-1 visa, you have 90 days to marry your partner after they arrive in the U.S. What they leave out is that after you marry, your foreign partner is not instantly awarded the visa. You still have to go through the difficult immigration process and prove to USCIS that your marriage is bona fide, which requires collecting and disseminating all sorts of evidence and attending an in-person interview with your spouse at a local immigration office.
I can't imagine most of these couples are successful in proving their marriages are legitimate. The K-1 visa was established during the Vietnam War so that members of the armed forces who met their partners while serving could easily bring them back to the States with them. It wasn't intended for down-on-their-luck loners to import con artists who they met on shady dating websites.