Many starfish not getting enough of what they need tend to visibly dwindle over an extended period of time as opposed to just showing up dead one day. So, you probably could "get away" with keeping it for a while and thinking that you're doing just fine keeping it. These particular stars may not be impossible to keep, and may actually be one of the hardier stars, but definitely need a tank bigger than 10 gallons.
The reason fish stores carry things that may not survive/thrive in home aquariums or that are environmentally irresponsible is because people will buy them (usually without doing their research first). For instance, a local reefer I know found a crown of thorns starfish for sale in a florida shop. First of all, those things eat coral, so unless you can supply it a steady diet of coral it's going to die (which, for the reefs, may not be a bad thing but is certainly not humane). Second, as a potential invasive species threat, they shouldn't even be exported to begin with.