IANAL but from what I understand, failure to enforce your IP in situations like this is often legally interpreted as a forfeit of your IP, and creates much more friction if you ever need to enforce it down the line
Sure, but giving them a generous IP license isn't considered a forfeit of your IP.
Blizzard actually went a step beyond and asked for your forfeit of any IP that you make with their tools. Not that any of those tools was used for Turtle WoW.
