You would notice also that the attitude that scripture encourages chrisitians to have is diametrically opposite to the attitude that JWs have. That scripture shows that christians don't need to go through the trouble of finding out the origins of some seemingly harmless custom or practice to determine whether or not they should participate. By contrast, JWs are taught to have the attitude of first finding out if some seemingly harmless practice or custom has some pagan roots or not before they participate.
JWs worship a God who stigmatizes harmless activities if there is any remote pagan connection to them in their distant past. But the scripture you quote shows that God does not stigmatize food or practices based on their past or origin. If he did, then under no circumstances would it be right for a christian to eat meat that was once in a pagan ceremony. But Paul's words show that a christian can eat such meat if his conscience and the consciences of others permitted it due to ignorance. Just as there is nothing wrong with eating meat sacrificed to idols in a non-religious, non-pagan context, in the same way there should be nothing wrong with celebrating a birthday if it's not done with any religious or pagan intentions - once the consciences of others won't be adversely affected.