I used to think this way (that the law is imposing upon divorced parents to pay for education and not married parents), however there is another way to look at it.
If both parents don't want to pay for their childs education, they don't have to, divorced or married.
If divorced, one parent can force the other to help pay for the child education.
However if married, and one parent wants to help pay for their childrens education, they can and by association is spending the assets of the other parent.
So if you look at it that way, there is no difference between married and divorced when it comes to being forced to pay for post secondary.
Interesting way of looking at it.
Not all marriages are structured with joint accounts. Many have three accounts: A separate account for each person, and a joint account for joint expenses that is funded (often proportionally) by both partners. If one partner in this situation decided to unilaterally pay for a post-secondary education, they would probably lose almost all of their discretionary spending. This allows a couple to make some independent financial decisions, while other decisions are made in a more cooperative manner.
Some marriages are completely financially joint. In those cases, your analogy is stronger, though in those marriages spending decisions are usually not unilateral. Joint accounts are usually held by people engaged in joint-decision making.
In a divorce, the financial structure is closer to that of a marriage with 3 accounts: 2 separate and 1 joint. Parents can engage in unilateral decision making up to a point (eg. I'm going to serve my kid filet mignon every day for a month) but that hurts their own financial position without affecting the other parent.
In divorce, the situation is aggravated by the fact that the parents can have wildly different financial positions. One parent could be wealthy (perhaps through remarriage), and, under the current rules, can force the other parent to incur a financial hit that is unaffordable to the poor parent, but well within the means of the wealthier parent.
That said, I'm not directly rejecting your analogy, I'm just pondering.