Okay, with the clarification that we're trying to get them onto another ship that will then get them aboard the SS Plot Device...
Space travel is expensive
In a single star system, species that only discovered one another a few decades ago likely don't have particularly advanced spacecraft yet. We could therefore imagine that building a single interplanetary ship could cost as much as, say, the entire Apollo program. If you have a choice between spending the local equivalent of $100 billion to build your own ship to take you out to mine the asteroid belt, or spending $5 billion to get a seat aboard someone else's ship, you could well be willing to hold your nose for the duration of the flight and travel aboard somebody else's ship.
Space travel is dangerous
Two asteroids are being mined, one by the insectoids of Shmenus and one by the smelly apes from Shmearth. Due to a very slight miscalculation, it turns out the two asteroids are actually about to impact one another. Both mining colonies need to evacuate, immediately.
The nearby Shmartian freighter ship may not like the slimy insectoids, and the apes may keep making fun of the captain's feathers, but you can't just leave them all to die, so...well, I guess we'll have to pick them up. Just long enough to get them back to their own people, of course.
Space makes a great prison
Prison hulks are a longstanding tradition hear on earth, and a space-hulk to hold POWs from the first interplanetary war would be an extremely secure location. With prisoners from Shmenus, Shmearth, and Shmars, the Shmuropan squids are moving their prison hulk ships out to the asteroid belt, where the vile xenos will perform hard labour to work off their sentence after their monstrous aggression against sacred Shmuropa. Hey, it worked for Blake's Seven...