Knold, you are quite right that radio signals don't care about borders. That will be why the UK NOTAM says 123.45 must not be used for A to A comms within range of a UK ground station.
I think 123.45 has been used for ground stations in the past - don't know if it is at present - but, I presume, the frequency co-ordination process has ensured that it will not normally interfere with use by Dutch SAR activities. I presume also that the process ensures that the legitimate use of the frequency in other countries is similarly protected.
If you tx on 123.45 near to the Dutch borders you'll cause interference - I'm guessing that the frequency is not assigned to A to A.
Radio signals don't care about borders and nor does (in this sense) the frequency assignment and co-ordination process. At least, it's not supposed to!