Its not about glass bottles. It is about the nature of the tax. Capacity tax is imposed on the production capacity of a beverage plant, as opposed to its production. Foreign beverages win, because their production capacities tend to be more or less fully utilized, because of the pepsi/coke craze in our country (punjab beverages makes pepsi). Local brands such as Pakola suffer, because while they may have enough spiggots, they cannot afford to produce at peak all the time, as their sales are significantly less. Hence, they end up paying taxes for stuff they are not even producing or selling.
Its more like a flip of how our IPPs are paid based on production capacity, as opposed to actual production. Issue is, that in case of beverages, many politicians have stakes in Pepsi/coke, hence they are ok with the imposition of this tax. (In a worst case scenario, they can always make their finance manager pretend to be the FBR chairman and go to court on their behalf)., And in case of IPPs, since they are on the receiving end of the payments, they again want the capacity payment structure. A win-win for politicians.