i am running out the door - so sorry for being so short, but ...
yes, this is very common w/many dress shops. they don't want you going in and taking pictures, then go and have a replica dress made. other than David's Bridal and some of the "once wed" places, i don't know of any place that will let you take a picture. it sucks when you're dress shopping and want to show pictures to those that couldn't shop with you! i was asked for the designer and style number, then i was able to find mine online - is that an option?
honestly, if this $750 dress is THE dress, and you can afford it (cut back elsewhere?) - i say go for it! make sure YOU love the dress too, not just that everyone else loved it. you don't want to regret buying a dress you don't love just to save a little $$ - this is your day, you're the bride!