Prosecuting the consumers (vs. the producers) of child pornography is an attempt to increase the risks of obtaining it, hopefully disincentivizing demand. This line of thinking and policy is akin to prosecuting customers (as opposed to the providers) of prostitution, or prosecuting users (as opposed to makers and sellers) of controlled drugs.

We can debate about how effective or ineffective or smart or misguided this is, but that's where it comes from, and is a frequent tactic to split the customerbase into two parts: the ones who may have been interested when the risks were lower and are now dissuaded, and those who are not deterred by the risks and choose to pursue it anyway.

Society can then point to the fact that everyone was given this choice to paint the latter group as not only morally corrupt, but also risk-taking, thoughtless, irresponsible, or simply people who "had it coming". This is a common way of providing for a permanent underclass in ways that promotes good societal mores, doesn't discriminate along ethnic lines, and doesn't disproportionately punish the economic poor.