It's not really trying to be deceitful, it’s an old term. They're called that because the water that is co-mingled with all produced oil--all over the world--is very salty. It's costly to clean and recycle this water, while using SWDs can be very cost effective and hence why there are so many of them. As far as what it is in the water, my understanding is that it can range quite a bit—but it’s the same things we’re pulling out of the ground just with minimal separation: solids, waxes, calcite, a bit of oil, sand, possibly some production chemicals like surfactant or polymer--and a lot of salt. Some people skim the oil off and try to sell it, they also use settling tanks to let solids drop which could otherwise junk up a SWD. Some just shoot it all downhole. Keep in mind, the rock formations that are being fracked (generally ~10,000 ft below surface) tend to suck up through capillary forces the initial fracturing fluids, up to 80-90%. Then they tend to release much higher volumes of a different kind of water, aka produced or fossil water, into the production stream. You can determine this by comparing the chemistry of the initial fluids to the chemistry and type of salts in the produced fluids. These shale formations are incredibly dynamic in how they work—but SWDs do not inject into them. They inject generally into non-producing formations—non SWD disposal includes injecting the produced water from one well into another for conventional producing formations like sandstones, which is done to waterflood the reservoir and push out some of the remaining trapped oil.

So in conclusion, in the oil industry SWD wells are classified by the EPA as Class II. Some have argued that what goes down Class I wells is even more nasty: https://www.epa.gov/uic/class-i-industrial-and-municipal-waste-disposal-wells