maurice I think the fundamental difference between the cost of a commute (or, my previous example of Super Bowl tickets) is that you, and you alone, can make the decision about whether it happens or not, and therefore influence and control the impact it has on your life as a couple.
If the length/cost of your commute was impacting on the quality of your DHs life (ie, limited time with his spouse), then you can act to change it.
It is likely that he is happy to financially support you, because he also emotionally supports you.
But for many stepmums, that isn't the case. They do not emotionally support the parenting decisions their spouse makes regarding his non-resident children. He may be a Disneydad, he may acquiesce to his ex's demands "for a quiet life" at the expense of the family, or he may be distant and unavailable to his DCs. He may not be pursuing more contact with them. He may overlook their violence or abuse in her home. In those instances, it's probably very hard for a stepmum to make financial sacrifices herself, in order to support her spouse to fulfil his financial commitment to his DCs.
A stepmum who financially supports her stepDCs is doing so to support her DH. It is not an a altruistic gesture to support the DCs - if it was, then the contribution would be irrespective of her marriage to their dad. If the stepmum can't emotionally support him as a parent, it's unlikely she will feel comfortable doing so financially.
What's clear from MN is that many, many stepmums don't discover the nature of their DH as a parent until they are committed to the relationship. Indeed, often the marriage itself, or the arrival of subsequent children, fundamentally changes the way in which the NRP parents those DCs. So, a stepmum who may have been willing to be both financially and emotionally supportive when she committed to the relationship is faced with a situation she was unprepared and unaware of.
The equivalent would be, for instance, if your commute suddenly involved something your DH fundamentally disagreed with. (Let's say, for instance, that you had to practice a particular religion in order to travel); would he be as willing to make financial sacrifices to support your commute then?