My answer to the first part of your question; No I don't feel ashamed... 
There are special vehicles for disabled persons and Prosmatec is not for them! This sticker is for the non-disabled Prosmatec drivers. And above all, it is humor, a satire. And Prosmatec owners around me understand it well.
Baqi meray bhai, agar khuda na khawasta mujhay pori zindagi iss sitcker k saath gari chalani par jati hai to mai iss mai Allah ki marzi samjhoun ga.
You didn't like it, you have your reasons and I respect them. I have put this sticker not to hurt anyone's feelings. Well, feelings of people who can't understand humor do get hurt irrespective of the fact they are disable or not.
Thanks for posting in and visiting my thread. Do visit again.
Regards. 