Ask HN: Is it ok to interview with other companies after accepting an offer? 15 years ago
The way you put it (and that's an important condition!), I wouldn't call it unethical. However, if you go to an interview knowing you won't accept the offer even if you get it, you're effectively wasting the interviewer's time (and there may be more than one person involved in assessing your skills). I was in a similar situation about 3.5 years ago when I scheduled interviews with three different companies, and I accepted the offer after the first one. I called and canceled the other two interviews for the same reason: I'd rather have those interviewers working with other prospects.