Many places in The United States have plasma donation centers. At the end of the donation you are payed for your time, because outright selling your bodily fluids is highly illegal in The United States. The penalties are much stiffer then a solicitation charge.

Everyone knows why they are there including the nurse, receptionist, phlebotomist, and donor. If you were truly being payed for just your time, couldn't you just sit in the lobby for a few hours without a deposit, and still get paid. I think not.

How does this differ from the hobby?

I know that blood may be donated but not sold to a collection center who in turn will sell it to those who need it; plasma however may be sold legally. Are you sure it was blood being donated and payment was made for time spent there?
What is the difference between prostitution and  plasma sale? I think prostitution is sexier? Only vampires get orgasmic over blood and they do not pay for it.....

Did the law on that change?

When and why?

I donated for about 3 years. Read the fine print they will tell you, you are only being paid for your time. It was nice when I donated. I weigh under 120 lbs, so I could get in and out in an hour.

a number of screening questions. Such as "have you visited a prostitute?". This was many years and I answered no truthfully. I got the feeling if I had answered yes my donation would have been unwanted.

Maybe the corporations buying blood are less picky than the Red Cross. Or maybe times have changed.