Good for you! I did almost the exact same thing at our Petco. And another time, I rolled up on a family where wife was holding a betta cup and they were looking at that display of cubes about same size as the cup. I asked very politely "you weren't going to buy one of those cubes for your betta, were you,"...and then ruined it by saying " because bettas should really have at least 2.5g to live in permanently,"...and just as I was sucking wind to continue rant that any thought of those cubes always seem to engender, the lady and her husband, give me the handle with care look, said, "oh no, this one is for my 10g at home. We know about bettas." Then it was all "come on kids, we're late" not thinking I saw teenage daughter with a betta cup and the one-up from the cube.
The betta cups have message clearly stating that it is unacceptable for permanent home and has Petco logo...but they sell the cube which holds, what maybe one more ounce than the cup they come in! Makes me so mad!!