I don't think you are being totally unreasonable.
And I think many people here are quite clueless about some other industries.
For example with traveling over the weekends, if the business trip is on different continent and there is 8 days worth of work, traveling every Monday there and Friday back would mean a lots of unnecessary trips, constant jetlags and would be rather unhealthy.
Not everyone is able and willing to spend that time away, but that is what job descriptions are for - some jobs come with more extensive travelling than the other ones.
Another hilarious idea is planning only for days this employee is there. That would effectively limit number of part time workers in a department to one or two. And, generally part time work isn't superior to all other needs, it is just one extra variable.
In this case the employee was allowed part time work with mutual understanding that there will be some switching to meet business needs and education might be a valid case.
One issue might be communication. Maybe it wasn't clear that you consider that event as the important one to use up one of the swaps.
When I was part time, my manager would tell me that some event was happening and what was the "mode" - sometimes they would express that if I would manage to swap, they would really appreciate it. Sometimes I was told I'd be welcome to be there and swap days if I wanted to be there (but it wasn't necessary), sometimes they told me not to worry about that one.
In your case we would discuss which of those two days were more important and whether I would be able to join, but it would be a discussion, not a silent expectation.