The Forerunners were most likely studying the flood because they belived that there wuld be something to gain from it. I meen flood are like superhumans they can jump extremely high, are almost immune to bullets and need only one host to spread out again.

The Flood found on the rings were kept there to study, the Forerunnners hoping to gain something from there stuys knew it was very risky and created the rings as a fail safe. If something went wrong and the flood got out of control they'd run like hell to the shield worlds and the reclaimers would set off the rings eliminating all sentient life with enough biomass to host the flood.

But to target these sentients you need the index, so y not change the index to target something else....