You're confusing the two things entirely. 'offering' and 'accepting'.
Ok, when I offer to treat my friends it's actually a treat and I don't expect them to pay me back. I agree with this statement, I don't treat my friends and family to be given things in return, I do it because I enjoy doing it.
After seeing this post they, or others, might no longer feel they can accept friends' generosity if they can't pay it back. I also don't accept generosity if I'm not in a position to, or have no intention of, paying it back, it just doesn't sit right with me.
Most relationships in life are reciprocal in some way, not with money or things, but time, effort, support, whatever.. if one person only ever takes and never gives anything back the relationship is pretty much doomed, there are plenty of these types of people about who think the world owes them something.
The whole point is, OPs friends ARE in a position to return the favour, but they CHOSE not to in a very awkward way. IMO they should not have accepted using OPs voucher. And if they had no intention of returning the favour (personal choice), then they should have used their voucher when they were alone, not invite others out by TELLING them they had a voucher to use, let OP pay for things they haven't even had and then make a show of not including them in the money off.
Takers only think about themselves, they will probably be very smug that they got OP to pay for some of their meal, and also got to use their money off voucher all to themselves, win win.. completely unaware that they have caused a rift in that friendship by being so discourteous.