I offer my explanation, since none of the answers below seems to explain the solution.

The probabilty is indeed 1/2. There are two things to realize:

1. The probabilty that Steve chooses his assigned seat is equal to the probability that he chooses your assigned seat.

2. In case that Steve would choose neither his own seat nor yours, then there are two alternatives: if somebody else would choose Steve’s seat at random, then you would get your assigned seat; otherwise you would be left with the Steve’s seat.

With that being said, we can go on to find out the probability. With every person choosing a seat at random (including Steve), there are there possible outcomes:

1. either he chooses your assigned seat, or
2. chooses the Steve’s seat, or
3. chooses someone else’s seat.

Notice, that the probabilty of choosing Steve’s seat is always equal to probabilty of taking your seat. That means that the probabilty of you getting your seat vs. not, is even. The case of a passanger choosing someone else’s seat doesn’t affect your final outcome in either way, it just passes that three possible alternatives to the next passenger.

Since the probability of someone taking your place is always equal to the probabilty of someone taking Steve’s place (and this also applies to the penultimate passenger with only two seats left), the probabilty of you getting your assigned seat is in the end 50%.