Paul Verhoeven was very influenced by The Terminator and wanted to capture that same feeling when he directed Robocop. I'm guessing that he used the score because he wanted to attract the same kind of audience that liked the Terminator to Robocop.

From an interview Paul Verhoeven gave in 2002

The Terminator surprised me, for example – I was taken aback by that. It was like ‘wow – that’s somebody really who really thought about something! And really did it, with cheap means, relatively, and made something very original. If anything influenced me when I came to the United States, it was that movie, because Robocop is not derived from Terminator, but I studied it very very well before I embarked on Robocop, because Robocop was of course still very outside my Dutch genre. I didn’t know much about that kind of film-making. So I think The Terminator was a very good education for me – from a philosophical point of view it’s an interesting, audacious statement. Yes, it’s science-fiction, but not completely.

Both films were distributed by Orion films. So that suggests he had easy / cheap access to the score from Terminator. So watching the trailer, and hearing The Terminator theme music could subtly suggest to his target audience "hey, this is a film that will excite you the way the Terminator did."