It's not a generalized warning at all. Most sand (esp pool filter, play sand, concrete sand) is composed of silica (quartz). Thus the belief that it's not the best choice for aquariums as diatoms and some algae feed on silica.
Long term exposure to the dust is the issue, not buying a bag and bringing it home.
Silicosis is a real disease.
If your bag doesn't have an MSDS on it you can bet there was one either attached to the pallet, included with the bill of lading or identified somewhere. OSHA tends to take the MSDS rather seriously.
Some additional info:
Silicosis in Construction
http://www.osha.gov/SLTC/silicacrystalline/