Let's not think of that as a disagreement, merely a misunderstanding. I AM with you on many points, and there is one observation I just made that always makes me smile since it seems such a constant through all the years I've visited forums - and I know this feeling very well since I felt it too for a long time!
Namely, the constant fear that people will ask WHY someone buys balloons.
It's our own shame talking, I guess. The trepidation of being left speechless. Maybe of not having a good excuse handy. The reasons are many but the fear's the same. I think THAT is a big part of why many call for better visibility, so they wouldn't have to live with that silent, constant worry of discovery.
The thing is that party store clerks want to make a good impression, appear like they take an interest - and since many people who are planning a party look forward to it with joy and a certain pride, asking about the usage is a good way to break the ice.
In other words, they ASSUME you're having a party or other function.
If you want to have an absolute truthful answer that works wonders, try "Oh, it's just for a small celebration. Nothing special." After all, it IS a small celebration of your own sexuality!

Should they still press for details, give them a smile and say "I've been asked to keep it a secret" - which, as of this post, is another absolute truth.
That mindset helped me get rid of MY shopping anxiety - your mileage may of course vary, but remember that no one will assume you're buying them for anything but decorative purposes unless you actually enter wearing a shirt saying "I Hump Balloons" - and if you're brash enough for that, you probably don't care what they think either...
I've watched forums for other kinks and the same problem is evident there - since the fetishist himself (or herself) is so very, very, aware about what the object in question is going to be used for, it's easy to think everyone else will be too.
They aren't, unless they share the interest - and in that case they won't be judgemental.