How and why is it legal for an adulterous wife to get money from the divorce?

The law in many western countries considers marriage to be a particular form of economic union. Most property acquired during the marriage accrues to the benefit of both spouses, as do their debts. If the marriage ends, each spouse is entitled to 50% of the value acquired during marriage (less any debts acquired during that time). Many jurisdictions now have no fault divorce, which means it doesn't matter why the marriage ended, nor does a court attempt to assign blame. It's over, and the assets of the marriage get divided up. And since the law applies equally to everyone, wives are treated no differently than husbands. So the cheating husband doesn't get any more or less either.