Why are magnetized materials dangerous goods?
98% percent of magnets - the everyday ones - are completely harmless. I believe the concern is with super-strong rare-earth magnets, where they can be problematic. Aside from Simon's valid point on interference to compasses:


Getting stuck to things, such as other bags or the infrastructure itself, in the baggage system.
Attract themselves to other packages and content, possibly damaging these. Think of hard drives and electronics.
Being a nuisance and safety concern should somebody decide to open it, such as customs or security. You can get your hand crushed in the process if you open the box and the thing springs out.


Be aware they do not appear to be banned - just regulated in that they have to be properly labelled and packaged for safe transport. More curiously in that list are 'First Aid Kits' and 'Seatbelt Pretensioners'.