FTR, it's been pointed out in a report that this was revived, but for now I'm treating it sort of as an official topic since it will be replaced at some point with one. (Whenever I get around to it. )
Magnetism vs. Gravity -- the difference -- and it's not a physics one that would directly answer the question but it can explain the difference psychologically -- is that Gravity is a natural part of everyday life on the large scale. Magnetism is not. Magnetism IS operating inside and between atoms constantly, but on the large scale it's odd. So absorbing it is not likely to cause major problems. My guess is that the GBs put an artificial limit on Gravity Toa for this reason, and also (a fallback reason if that's not true, or an addition reason if it is) so that they wouldn't be overpowered considering Gravity has some advantages already no other does.
Note that the GB reasoning for such limits would have been against the risk of a Gravity Toa glitching and going berserk, rather than using full-sapient freewill to choose evil, but the effect is the same.