Which is odd in HGSS's case since they didn't prevent a Pokemon from evolving into a Gen III or IV Pokemon. And it's not like there isn't locations they couldn't put them. Put a Ice Rock is the Ice Caves (and if they wanted to make it a post game thing have Rock Climb needed to get to it), Moss Rock could go into Ilex or Viridian Forest, and as
Vader_the_White said the Power Plant would be the perfect spot to have magnetic waves (or Lake of Rage where Team Rocket was using radio waves to evolve the Magikarp; they could say there's a small amount of electromagnetic radiation left which, while not harmful, will have Magneton and Probopass evolve).