The best explanation I can think of for this is simply the bias of Frank Norris. This influence in the book is light, almost unnoticeable, but when I realized that something in the book was making me read the situation exactly opposite of how I would normally do it, I had to look deeper.
My best guess ties back to naturalism. Perhaps, in Norris's mind, avarice is unnatural and is therefore the greater crime. In the book, there is something intensely abominable about the way Trina saves her money for the love of it, despite living in squalor, and Norris continuously reminds us of this by telling us again and again of how she was obsessed, how she lied to her husband, how very deviant her love of money is. He illustrates this with Zerkow, who is stark raving mad over gold. Although Trina identifies herself with Maria, it is really Zerkow that she resembles.
McTeague, however, seems to be driven to his sadism by his wife. Norris insinuates that it is really Trina's fault that he is like that; he shows absolutely no remorse for torturing his wife - nor does Norris indicate that he should. According to the book, it is Trina's fault that he starts drinking whiskey and it is her fault that he abuses her, because he is abusing her for withholding money.
This is, of course, as nasty a subtext as the stereotypical, moneygrubbing, murderous Shylock that Zerkow represents, but it is much more cleverly slipped into the book.
Not that I didn't like the book, though. I really enjoyed it. But I was surprised at how easily Norris's writing was able to insinuate his own bias into my perception before I could notice it.