From Diablo Wiki
WTF is with all the ladder only runewords?!?!? That sucks. And what difference does it make what kind of character you play? Does it really serve anyone to allow a ladder character to make and use a particular runeword and then prevent someone else from making and using the same word? Who makes this crap up?
- Ladder only runewords are Blizzards way of getting people to play on the ladder realms. It makes the competition for the ladder more popular. Blizzard did that with the ladder reset/update for 1.10. The ladder runewords are pretty good, and that is why before a ladder reset, you want to make a lot of ladder runewords on ladder, so that when the ladder resets and your ladder chars become normal realm chars, you have a lot of runewords that are rare for normal realms. Blizzard makes this up in order to get players on the ladder realms.