Law always puts the hand releasing his Devil Fruit powers in an uncovered position instead of hiding it or even protecting it with Hardening Haki.
And since he has hax abilities which are difficult to counter, there's a high chance his opponent will choose the most effective and easiest way to beat him : attacking the hand unleashing his Devil Fruit powers.
For example, Zoro can be pretty good at assessing his opponent's strengths and weaknesses (like during his fight against Pika). So he might come up to this conclusion and decide to throw a long-range slash to cut off Law's hand and render him unable to use his Devil Fruit powers.
And since he has hax abilities which are difficult to counter, there's a high chance his opponent will choose the most effective and easiest way to beat him : attacking the hand unleashing his Devil Fruit powers.
For example, Zoro can be pretty good at assessing his opponent's strengths and weaknesses (like during his fight against Pika). So he might come up to this conclusion and decide to throw a long-range slash to cut off Law's hand and render him unable to use his Devil Fruit powers.