300CPM above background is considered very low - likely why they classified this as non-emergency - the only reason it was reported was per NRC cfr that states any time there is transportation of a radioactively contained person offsite, it must be notified.

For reference, in Canada, that is considered trace contamination and not dose. You would experience 300-800 CPM on a commercial airliner during the entirety of your flight, for comparison.

edit: adding to this that the site in question, Palisades, is shut-down and is under decommissioning and was not operating at the time - so while the water would have had some radioactivity due to exposure to the formerly active core, it was not like falling into an operating reactor or into moderating heavy water... also something that cannot happen with a pressurized reactor such as this one.