This has been discussed to no end in the past. When I published my paper, I did so in a venue that had open-access to all its proceedings (HotPower 12). However CODASPY where this paper comes from isn't one of those venues. ACM explicitly denies permission to host the papers elsewhere - From the paper:

Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than the author(s) must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee.

While we might question the ethics or morality of closed-access conference proceedings, it is definitely on the wrong side of legality. Much like patents are evil but can be defended/enforced by big corporations legally.

IMHO, professors and students who take public grants should publish only to open-access conferences and journals. If they do not do so, it does not justify illegally downloading the paper.

Would you accompany a movie recommendation with a piratebay link on HN?