Being told by who? Your PTA district or your school district?

If you are truly part of the PTA (Parent Teacher Association) and not just an independent, generic group, then you must follow the PTA rules. I don't believe it's possible to have a PTA chapter without a membership fee.

That association is a formal organization with state and national presence, including paid employees at the higher levels. The way that it exists/pays for itself is with member dues. Whatever the membership dues amount assessed at a Texas chapter level, $4.50 of that goes to State and National. ($2.25 each). So you might be able to make your LOCAL fee zero, but you would still need to charge $4.50 to cover the state/national.

If you were an unaffiliated group, such as most PTOs (or whatever name you choose except PTA), then you would not be accountable to another organization about your dues structure and could have free membership.

I am the president of a PTA in Texas and in the last few months read a great article about offering free membership to our families. We have done some research, run it by our principal and have offered this to our school. (one membership per household) We are now being told by our district that it is not allowed??? What am I missing here? Any help would be greatly appreciated!