Your capacity to presume or confect the existence of presumptions by other people is extraordinary !
I am not assuming a link between illiteracy and criminality, at all.
Let me spell it out for you.
The "lower class" ( for want of a better term, lest you start making presumptions about assumptions again ), were not much in the habit of placing ads and announcements in the newspapers. You will observe, in the newspapers of the day, many advertisements, announcements and random pieces of information about the gentry, the merchants, the officials, the professionals, the graziers and the trades.
You will see next to no information about the small-holders, labourers, and others. Many of these were former convicts, many were not. And among the former convicts, many had no track record of criminal misconduct in Australia.
In that period, the gentry, merchants, officials, graziers and tradesmen were generally literate, and many of the smallholders, labourers, workers and ex-convicts were not.
I see it as somewhat unusual, to have an advertisement placed in the newspaper, as John Spilsbury's advertisement concerning his absconded wife, which specifically draws attention to his illiteracy, particularly as there seems to be no obvious reason why it would be mentioned at all. I have read thousands and thousands and thousands of pages of these newspapers, and that still strikes me as somewhat odd. The point being, that being illiterate is not inconsistent with being a former convict. The use of the double negative in the preceding sentence is intentional and has semantic meaning. This should not be taken to mean an assumption is implied that illiteracy implies criminality, or that criminality implies illiteracy, or the literacy implies an absence of criminality. In fact, to say that A is not inconsistent with B, is to specifically deny making any of those assumptions.
Nevertheless, the fact that the ad specifically draws attention to John Spilsbury's illiterate state, highlights the unusualness of his ad, because most such ads were placed by middle class rather than lower class people.