That may be true. But we have to look at the actual nature of the sources.I searched thru the Indexes of of following books which are primary sources on Aurangzeb:
(i) Maasir-i-Alamgiri : The official Mughal record of Aurangzeb's deeds completed 2 or 3 years after his death
(ii) Storia do Mogor by Niccolao Manucci: An Italian traveler who came to India and settled here. Manucci lived thru the entire reign of Aurangzeb. He was employed in Mughal setups as artillery man (by Prince Dara, son of Aurangzeb. by and Mirza Raja Jai Singh, the topmost Hindu Raja who served in Aurangzeb's army) and as physician (to Prince Shah Alam, son of Aurangzeb).
Neither of the above talk about ban on cow slaughter.
(i) This was written by an orthodox Muslim who wanted to portray Aurangzeb as "the sword of Islam". So in his zeal, he tends to give all of Aurangzeb's temple destructions an iconoclastic motive, even though some were clearly influenced by other reasons. It is thus natural for him to omit laws by Aurangzeb that defy this "Islamist Emperor" perception.
(ii) Manucci had utter contempt for Aurangzeb, and even refused to enter the emperors service when his Agrah friends persuaded him in 1663 (when Aurangzeb went to Kashmir). So he did not have direct access to the Mughal court in the 1660s, and thus really had no way of knowing the detailed laws put forth by Aurangzeb. He was only employed with Jai Singh in the 1665 campaign against Shivaji. Then he abandoned the service and went to Goa. So most of what he says about the Mughal court in that period is influenced by public gossip...
Bernier was under the employment of Danishmand Khan, one of Aurangzeb's nobles. Being thus employed and a good friend of Danishmand Khan he had a good way to obtain detailed info about the Mughal court. He even quotes some of Aurangzeb's letters to Shah Jshan in his book.