I searched the text of Abaddon's Gate (the book in which the Ring Station is found) but I can't find anything that directly answers your question. The best description of the slow zone comes from the humans shortly after they first encounter it:
"The slow zone, based on the sensor data we’re able to get, is approximately one million kilometers across."
We’re calling this Ring Station, for lack of a better term. It appears to be a solid sphere of a metallic substance, measuring about five kilometers in diameter. Around it is a slow-moving ring of other objects, including all of the probes we’ve fired into the slow zone, and the Belter ship Y Que. The torpedo that chased us through the Ring is headed toward the station in a trajectory that seems to indicate it will become part of the garbage ring too.
But the most intriguing factor of the slow zone, and the one that gives it its name, is the absolute speed limit of six hundred meters per second. Any object above the quantum level traveling faster than that is locked down by what seems to be an inertial dampening field, and then dragged off to join the garbage circling the central station. At a guess, this is some sort of defensive system that protects the Ring Station and the gates themselves. Light and radar still work normally, but radiation made up of larger particles like alpha and beta radiation does not exist inside the slow zone. At least outside the ship, that is. Whatever controls the speeds here only seems concerned by the exterior of the objects, not the interior. We’ve done radiation and object speed tests inside the ship, and so far everything works as normal. But the last probe we fired was immediately grabbed by the field and is now making its way down to the garbage ring. The lack of alpha and beta radiation leads me to believe that there’s a thin cloud of loose electrons and helium nuclei orbiting that station as part of the garbage ring.
While the book does not directly answer your question, we can make some educated guesses as to why the Ring Station behaves the way it does.
First, it would take a really long time for any object in the slow zone to actually reach the Ring Station when you consider the large diameter of the slow zone (approximately one million kilometers) compared to the "speed limit" (600 meters per second). The Ring Station would have ample time to react if the humans tried to attack it from within the slow zone.
Second, the Ring Station's activation of the slow zone is indeed a defensive mechanism, but it's not the only defensive mechanism it has. The Ring Station is actually very powerful:
"I’ve seen what this station does to threats," Holden said. "Miller showed me, when I was there. All this slow zone stuff is non-lethal deterrent as far as it’s concerned. If that big blue ball out there decides us monkeys are an actual threat, it will autoclave our solar system...Something was attacking them, the protomolecule masters or whatever they were. Their defense was causing the star in any...infected solar system to go supernova. That station has the power to blow up stars, Naomi."
The Ring Station is not really threatened by any of the human ships in the slow zone because it can activate far more powerful defenses. It doesn't need to eject them or keep them frozen at a distance -- it can simply destroy any ship that tries to attack it further (e.g. with a laser, which would not be subject to the slow zone's "speed limit").
Third, by capturing rather than ejecting nearby objects the Ring Station can continue to monitor the potential threats posed by the ships, missiles, etc.