I'm middle-aged and thought probably around the age of most coaches these days, but the most recent waves of new coaches are probably younger than me and don't really remember the running boom and the generations of runners at that time. What I can tell you is that this was once normal in high school. I realized it's unheard of now, as the kids who do a actually run all year, consciously train to get better, etc, run both. So from that viewpoint, it is strange that current college-aged kids would do as you describe. Didn't every one of them run both like the year before last in high school?
Anyway, from the perspective of an enthusiastic teenage runner: the distance crowd back in the late '70s didn't run for the school track team. On of my training partners (probably the main one, if there was such a thing) approached the track coach senior year and was turned down for joining the team. Why this suddenly happened 2 months before graduating when he didn't previous years I couldn't tell you. But I can tell you this: I ran the most miles per year of anyone at that school, and this guy was next.
So, even though this guy ran cross for 2 months each year (so did another of my main group, so I was more on my own in September and October) it's not like he was out of shape. We ran all year, did a marathon occasionally, and didn't need any prompting to stay in shape. To contrast, the guys who ran track - I knew just a couple, so maybe am going out on a limb very slightly with generalizations - did not run very much year-round. They needed the motivation of team, coach, and organized practices to get out the door.
The takeaway is that in my experience those who ran for the official school team during every available season were the least good distance runners at school. I believe I had the school 10k record (still might) and the track team crowd didn't come close to the 4 or 5 best guys in my group. So, come cross season, who would you prefer? The guys who showed up for track on day 1 but hasn't run a mile since that season ended? Or the guys who ran a Spring marathon and figured they'd do cross for a change of pace.
I'm just saying: don't confuse not participating in something you organized with laziness or lack of fitness. Getting distance runners to run for you for even 1 season should be a victory. They could just run on their own all year and the school/you get none of the glory. Discouraging what they're doing now will just turn them away from your programs in general.