iPhoto'08 is Event based, it was the signifigant update they added.
Unfortunately that's how it works, every upload/import will be an event, you can "Split" after upload. Even if you try to trick it by naming an event the same as an existing event, it still counts as a different event based on the date.
I actually prefer it this way as it makes organization much much easier. Rather than uploading a bunch of 'unsorted' images into a lobby, they're organized. It's not much effort to move them around afterwards, at least to me anyway.
As the above poster said, keep doing it and drag and drop by either merging the events via events view or dragging and dropping in photo view.