I agree with everyone who has responded to this post in favor of the Bride: maybe you think you truly know what it takes to plan a wedding, however, perspectives change drastically when it finally comes down to your own, I promise. When the money is suddenly coming straight from your pocket, and all of these decisions are truly on your shoulders, it can really change how you thought you’d respond to certain situations. Trust me, there were a lot of opinions I had about weddings not too far from your own until I started grappling with my own budget, space, guest list, etc. Suddenly, once you’re the bride, you understand why people getting married make the choices they do! It is a really difficult job, attempting to plan the wedding of your dreams on a budget, without offending anyone. You can act with as much tact and politeness as you can possibly conjure up, however, at the end of the day, there will always be SOMEONE who didn’t like your flower choice, or your vows, or the dinner, or the wording on the invite. You can’t please everyone.
I think your good friend has handled this beautifully, and I think you are right: she probably purposefully did not put “+ one” on every response card because of budget and spacial constraints, but the fact that she’s willing to bend that rule for you says a lot about how much you mean to her.
Maybe what’s bothering you so much is that she is politely making an excuse as to why she didn’t list “plus ones”. You mentioned she had said she “didn’t think about it beforehand”, but since you can see right through that reasoning, maybe what’s bothering you is that you want her to be honest with you as to what her motives truly were (i.e. saving money, space, etc). Being so close to her, maybe you feel like she is patronizing you by being so polite, and saying that she just forgot, rather than admitting that she did it for other reasons.
Either way, she has every right to make the choices she did, she is not being rude, and your job is to be there for her to support her through all of this as her Maid of Honor. She trusted you with this position, and I think you should put this in the past, and enjoy this moment that you two have together as friends!