If kamma is a speculative view then why did the buddha expound it as a fact not as a speculation?
Because the Buddha, as a good teacher, taught according to the mental abilities and understandings of the listener, pointing toward his own teachings. Kamma and reincarnation beliefs were entrenched in the culture of his time. He was knowledgeable and well-versed in all of the major philosophies and religions of his time and place, and conversed with persons who approached him using their own lexicon and base of understanding to point toward his own teachings. For example, when asked by a "naked dog-duty ascetic" and a "naked ox-duty ascetic" where they would be reincarnated, he pointed out that as one practices, so one goes: each would become a dog or an ox. His own (transcendent) teaching was that "kamma" is intention
(cetana). Our mental, verbal, and bodily actions follow our intentions.