Because updates should never take away existing gameplay, and right now non-Horizons players can explore the same way Horizon players can. Now an interesting question is - do the probes replace current exploration gameplay or introduce new gameplay? At first I thought it was the former, but it seems like the new DS replaces the old ADS and DSS, and the probes are something entirely new (mapping POIs to our nav panel). If this is the case, then your point is valid IMO.
That was not mentioned but according to the statement that Beyond is accessible only to Horizons player the old ADS+DSS should remain in the game for the non-horizons players. To be honest, Horizons was released 3 years ago and it should now be the basic version of Elite. They should stop messing with mulitple versions it just introduce limitations to both versions.
That's why they have the efficiency bonus.
It's just a credit bonus feature.
And Exploration is not about Credits!