It was Namco's own decision to change this to Pac-Attack (or Pac-Panic) so it's not the case of stealing but rather for marketing reasons. The original arcade game was part of a series and actually did get an US release (plus an UK release judging by a Sinclair User review) but must have really flopped in the arcades outside of Japan. Wonder if anyone played an English version of this in the arcades back in the day? Sega and Nintendo did the same for their puzzle games.

Namco was in a bit of mood of changing games to Pac-Man like what happened to Fury of the Furries became Pac-in-Time. Same reason.

Namco stole from themselves and made this Pac-Attack.