Doesn't a list of public proxies encourage net abuse?
No more so than a list of hardware stores encourages the abuse of crowbars; like crowbars, there are plenty of legitimate applications for proxy lists.
One good example is the use of such a list by online merchants, to cut back on fraudulent orders by denying access to known proxies.
Another example is the incorporation of proxy lists - including ours - into DNSBL zones, to filter mail which has a high spam potential.
The same technique could be used by just about any server admin to stay one step ahead of abusers.