Arhat kevali are omniscient, knowing everything that can be known and nothing relevant is outside their preview. They are without any sort of attachment or bias (Veetrag), do not have passions at all and have perfect conduct without any flaw. Such perfect ones attain omniscience. In the course of their endeavour on path of salvation they meditate, ponder over all aspects of life, analyse, synthesise and evolve and follow the rational path. They cover and grasp the whole spectrum and are thus omniscient. This is logical and should be accepted as such.
The problem is that omniscience has been over stretched so much so that destiny and all activities (Paryayas) of all animate and inanimate objects have become as if pre-ordained. They are attributed the omniscience of all future activities good or bad. Whatever was done being done and will be done by a particular soul in entire endless span is already known to them. An Arhat Kevali millions of years ago had already known entire future of everybody. This knowledge of kevali cannot be wrong; whatever they have known must happen. A particular individual got the opportunity of Arhat's audience and asks his future and is told that be will commit murder at some point of time in future. He repents and wants to strive to follow good conduct but he cannot because omniscience as interpreted has already pre-ordained the entire future conduct of every one. This is against the basic principle of Jainism that by following rational trinity by one's own determined effort (Purusharth) one can attain salvation. Effort and action (Karma) good or bad are in one's own power and non-else can interfere. The one god theory attribute all actions of every one to the will of one supreme god and thus all souls are thus puppets dancing, performing on the strings being pulled by the god. Thus no body is responsible for anything good or bad. If kevali are attributed to know all future actions and whatever has been known cannot be altered, then it is almost the same thing as universal control of one god. There whatever done is by his will and here whatever done is as has already been known.