Mike is there any specific reason that steam is forcing you guys to be listed as "free to play"? Pinball FX is not in that category and has the same "Free Core Game" "Table DLC" structure as TPA. They are able to sell individual tables as well as table packs without issue. That "free to play" listing seems to be more trouble than its worth.
It's almost certainly because of the Pro upgrade system. Pinball FX2 is pretty simple: you buy the table or table pack and that's it. With TPA, because you can elect to buy Pro upgrades individually per table, that greatly complicates what they have to surface to the user in terms of buying options. They're probably wary of people trying to buy Pro Upgrades without owning Tables, or people getting confused about what they bought, etc.