If it's for scrap it's barely worth the effort/fuel. If it's a landowner with a grudge what landowner would have so much of a grudge to drive between the Slate, Penmaen Head, and Ty Newydd multiple times (a distance of 50 miles at a guess) just to nick one or two lower-offs. The most likely explanantion to me is it's a climber stealing them for their own use but then where are they using them - we know all the new crags being developed so unless it's outside N.Wales I'd have thought it'd be obvious if gear was being recycled onto new routes?
Whoever's doing it is a timewasting prick and I don't really climb the routes it's happening on, hope they stay away from the ormes, lots of lower-offs there!