I don't know if that behavior was common in the 40's-60's amongst bosses, but Robert Kanigher was often accused of doing similar stuff to Morty. Only Kanigher was a writer, and didn't need to steal plots from one freelancer then give them to another, like he came up with them.

You really have to wonder why Shooter didn't say "forget this," and never take a call again from Morty.