Basically, these products have the potential to affect blood pressure, be it upwards or downwards.
It could be because if you are hypertensive, you have a decreased ability to deal with hypertensive stimuli an a product such as this could be held responsible if you were taking it while something bad happened, such as a stroke for example.
Maybe, such as Carbolin 19 and HOT-ROX from what I remember have some vasodilatory effects that could decrease blood pressure and we have no way of knowing what the effects of that would be should those be mixed with hypertension medication. It could be synergetic and massively decrease your BP and have you fall down and what not! I doubt it but people have claimed far more insane things from their supplements or medication.
So since there has been no medical conscensus as to wheter or not these supps have sufficient impacts on BP to warrant warning labels, it is better for a corporation to mention it on the label for safety purposes (yours and theirs-legally). In essence, it is a : We don't know so don't use it kinda thing.