Hmm.... Are you using this
http://www.petsmart.com/global/search/search_results.jsp?art=false&In=ALL&N=2030148&Ne=2 or this
http://www.petsmart.com/global/prod...um+salt&In=All&previousText=aquarium+salt&N=2 ? The first page includes products which you use to turn fresh water into sea water, suitable for a marine tank, and in a more diluted form, into water for a brackish tank. The second page has a product which, although it may be extracted from sea water, will not make sea water when mixed with fresh water, no matter how much you add. It will not make brackish water either, since brackish is a mix between fresh and marine. It never states that it can be used in a marine aquarium, and all the products on the first page do.
Since brackish is dilute seawater, it stands to reason that you would have to use the same type of product that you must use to make seawater, doesn't it? There are other minerals in seawater besides the salt, and aquarium salt only has the salt. The salt mixes for marine water include the other minerals as well.
If you have the product on the second page, I have the same thing; and on the top flap it says,"Professional Aquarium Salt for Freshwater Fish".
Unfortunately, pet-store training can't cover all the bases. We usually get a better result by researching what we are told there and making sure they are correct instead of relying on their knowledge.