For big tanks, there is a VERY valid reason. Non-tempered glass won't sustain the weight. Keep in mind that water weighs about 8 pounds per gallon. That's a lot of force. For smaller tanks, it's not as big a big deal, but it's still something to keep in mind.
Personally? No way would I build a tank to put in at my office. If it breaks, who will be liable? I'd hate to lose my job because a tank broke, especially if I built it myself.
In terms of not selling you something, pretty much ALL stores retain the right to refuse service for any 'non protected' reason. Refusing because of gender/age/race/sexual orientation, no. Because they think you're trying to do something unsafe, well within their rights if they suspect you will hold them liable in case of failure. If you really push it, the manager would like step in, but at that point, is it worth it?