Oga's doesn't have a virtual queue, boarding pass like the other SWL attractions or a FP+ option. They want/need to control the flow of guests in the cantina so they were forced to use the existing ADR system, even though it doesn't work the same way as it would for a full service restaurant. The fact that they need a controlled system is due to the popularity. The other pubs/bars in WDW that we frequent are simply not crowded enough, in such demand, or as popular as Oga's is to warrant any sort of ADR system.
Right now, this is not your run of the mill pub like Rose and Crown or Raglan Road's Hole in the Wall or Tonsil Tickler bar. I've never seen anyone in any of those places who didn't order something. I have seen people turned away from Biergarten when they asked to get a table to see the show and not eat. Every so often on the Dis people will ask about that also. Your ticket to Epcot doesn't entitle you to go into any restaurant and hang out. Even though Oga's is technically a bar, it falls into the ADR category for a reserved time slot. You're not making an ADR for viewing, it's with the assumption you will be ordering something.
Someone mentioned in the thread about going with a group of people and being the only one to not order...which is fine because it still creates sales for the table as a whole. The responses on here have been under the assumption that the OP was going on her own and not paying. And that is not something included in your admission ticket.
There is plenty of theming in the whole land, and all of WDW to enjoy. No need to assume that your entry ticket gets you access to everything. Like it or not... there is an exclusivity for people who spend more, whether it be for a dessert party, Oga's, high cost buffets, holiday parties, etc. Not everything is accessible to everyone who purchases a ticket for entry. Why do you think the character meals have a set price? Or BOG went to Prix Fixe? Because they don't want people just coming in for the experience without paying. It's just a sad fact that "extras" cost more.
I mentioned before that I wouldn't feel comfortable not ordering anything. I do have reservations for Oga's for May and already have an idea of what I want to order. It never crossed my mind that if I made an ADR that I wouldn't order something. To each his/her own.