On the recent trip out a couple of the adults got in a strangers car
Two vulnerable adults were left unsupervised for long enough that they were able to do this
and now the boss has said there are no more trips because of what they did?
Not because of what they did, but because of what they were able to do. The fact two vulnerable adults were under such poor care from whoever was supposed to be looking after them means that, rightly, the quality of care being provided by the day center is not appropriate and therefore no more day trips should be allowed until a full investigation has taken place to establish how this was able to happen in the first place.
Is this harsh? The adults that did this were Ok by the way?
No, not harsh, sensible. They may have been okay but that was purely down to luck. They were at risk, specifically because those who were responsible for their care failed in their safeguarding.
Your lack of understanding as to why this is such a big deal is concerning. Vulnerable adults are "vulnerable" for a reason and therefore require additional support to stay safe. If this can not be ensured by the people responsible for their safety, those people can no longer be trusted to take the adults on day trips.