It is no error, phenyl-2-nitropropene is beta-methyl-beta-nitrostyrene so calling it a beta-nitrostyrene is just being vague, not wrong. You could say "nitroalkene" too, being even more unspecific as to exactly what compound is being discussed.
Just take a look in Pihkal, where Shulgin refers to substituted phenyl-2-nitropropenes as "nitrostyrenes" all the time.