Yeah you're probably right. But atm there is no financial gain for doing so, as all copies I have seen for sale end up selling for the same, if not less than the US version without the sticker. Maybe in the future that will change, but right now, if you find one it was almost certainly distrubuted by Konami in Hong Kong/Asia.
My guess is Konami just used a sticker for this as it was an earlier release, but as Gradius came later, they had in house box production up themselves at that point, and so could print a proper box. Or something like that.
I'd be interested to know what PS1 games you are talking about. I have quite a few Asian PS1 games myself, but don't have any with a sticker on them. Most of them got a specifiic Asian release, that come with both a Japanese, and Chinese/English manual.