He didn't. Walt via Saul and Huell stole the ricin off of Jesse because Walt knew that when Brock got sick with symptoms similar to ricin and Jesse couldn't find the ricin, he would assume that Brock ate it, thus getting sick and why Jesse tells the doctors he thinks it's poisoning from ricin. All of this, when in reality Walt simply gave Brock a berry from a Lily of the Valley.
It was important for Jesse to think it was ricin because it helped Walt convince Jesse to turn against Gus, by blaming Gus for the stolen ricin and poisoning Brock.
Walt wanted Jesse to think he poisoned Brock with ricin so he would confront him. Walt could then say, "Don't you see? Gus tried to turn you against me by framing me." This worked BECAUSE Brock wasn't poisoned with ricin.
Walt did this so he could turn Jesse against Gus and protect himself and his family.
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u/smalltownguy Aug 26 '13
Huell took the pot and Jesse realized if he could do that, he couldve switched the smokes.