I'm not a Plasma donator but I do regularly give Whole blood. I would assume working out the day before would leave a lot of waste toxins in your bloodstream that they would not want. Same effect on women that are breastfeeding. Apparently working out make the milk taste bad or have waste toxins in it.
I have noticed that after giving blood, I am pretty much worthless as far as the intensity in my workout the next day.
And as far as cardio, I'm dead after just a few Min.
At least with giving plasma, you get the red cells back.
With Whole blood donation, you loose a lot of Red cells and that is why It takes me a couple of days at least to get back up to normal.