Interesting... I bet a lingot that it went down like this.
A. Spanish speakers (ancient diplomats?) learning dutch sometimes mistakenly use spanish you "tu" saying "alstublieft" B. People are generally too polite to correct someone as they are asking nicely for something. Alternatively, if you are in the middle of asking for something from them you don't want to stop to give a language lesson, so you just say 'please' the same way the listener does (so that they are less irritated). C. Over many many years... 'Alstublief' becomes the more diplomatic way of saying please.