It may have meant the absolute, because the term was likely invented by a marketer, but in reality "stainless" steel has never been rustproof. It really depends on the alloy, plus the manufacturing quality. Lower grades with less chromium rust pretty easily, just not nearly as much as regular carbon steel.

Another fun fact about "stainless steel": it's usually said to be non-magnetic: you can't stick a magnet to it. But again this isn't completely true: the lower grades are indeed magnetic, but the higher chromium-content ones are not.

Why bother with a low-chromium-content SS? Probably either cost, or performance reasons. The high-chromium alloys are more brittle.