It appears that bear canisters are recommended throughout the Parks (Sequoia and Kings Canyon), but are only required in 3 areas of the Parks. The following is from the "New Wilderness Food Storage Guidance" publication that can be obtained from the SEKI website.
"In 2009, these parks are strongly recommending that all campers carry all food, garbage, and toiletries in a park-allowed, bear-resistant food-storage container (canister or pannier). As the parks continue their efforts to remove broken and unused lockers from the wilderness, previous locations of food storage lockers may not be guaranteed.
NOTE: This recommendation is in addition to the existing requirements to carry and use park-allowed, bear-resistant food-storage containers in three specified areas: 1) Rae Lakes Loop and vicinity 2) Dusy and Palisades Basins and 3) Rock Creek drainage."
This sort of sums up why they are required:
"Through the years, portable canisters were designed, tested, and made commercially available. In 1991, only 1% of wilderness visitors carried these canisters. This increased to half of the wilderness users by 1999; even more use them today. While this trend is promising, human-bear incidents and injuries continue, leading these parks to require portable container use in a few areas starting in 2000. By 2003, portable containers became required in the Rae Lakes, Dusy Basin, and Rock Creek areas of Sequoia and Kings Canyon National Parks.
With these requirements in place, incidents have continued to decrease. Despite that, in the past ten years in park wilderness areas bears seeking food have injured nine people. Three bears were put to death due to safety concerns related to their persistence in trying to obtain human food."