This occurred before HIV was isolated and identified as the cause of AIDS. There was concern earlier on that there seemed to be a correlation between the platelets products and AIDS cases but they had very little data.
Obviously, I don't know the details of 1970s/1980s plasma products, but I think it's probably fair to assume that they were screening for pathogens they knew about at the time. I think in retrospect it's easy to say that they should have taken the factors off the market sooner, but at the time it was probably a very difficult situation. Hemophilia is an incredibly dangerous condition without treatment and there was a reasonable expectation that people would die without access to clotting factors.
Edit: To be clear, I know you're not making excuses for them either.