Bella is a shield,which means she can block powers that have to do with the mind(Edward,Aro,Jane,Demetri,Kate,etc).Alice's power has nothing to do with the mind.Alice can see Bella's future because Bella decided the course of her future when she learned Edward was a vampire and wanted to be one too.If she changed her mind,then Alice wouldn't see Bella's future anymore.I hope that explains it
Bella's power is a mental shield. No powers which affect the mind can affect her because it protects her (except for Renesmee because she has the ability to bypass at least some shields), but ones which have a real physical effect can. Edward and Aro can't read her mind because mind Lesen is obviously mental, Jane and Alec and Kate can't use their abilities on her because they're all mental illusions. Alice can see her future because she really sees futures. Benjamin can affect her because he really manipulates the elements. Jasper can affect her because he really senses and changes people's emotions.