I don't think my question needs to assume that - but I take the point.
But I can defend such a thing - the moral obligation to obey lies in the force of my argument. You might disagree, in which we case there lies a brute clash of fundamental ethical principles.
Firstly, everybody is born into disadvantages. I'm disadvantaged relative to somebody who was born to a footballer (and, likewise, I suspect they'll be disadvantaged in various other respects compared to me - they might not have as loving a family as me, and such like). A person born unhealthy, depressed, ugly or otherwise might be disadvantaged. Intelligence, good-lucks, and so forth, are dispersed unequally through the lottery of birth. It is foolish and mistaken to pretend that a few humans operating from a capital city have legal levers and institutions that they can use to make all these things disappear. They can't. And, indeed, central planning often makes the situation much worse (and this has been the spirit of much economic through since the likes of Hayek and the fall of the Soviet Union). But, you know, so what? I have the liberty to plan to overcome the hurdles I face in the way of my dreams and aspirations. And liberty is the best gift a government can give a person to solve their problems. Life is unfair, but why do we assume it's the government's job to make it fairer? There's more of us than there is of government. Be the change you want to see. We plan for ourselves better than the government can co-ordinate our lives.
Secondly, I'm a Rawlsian. I have fairly commonly-accepted moral axioms supporting my political position - based on formal equality and the golden rule. By asking: "what socio-legal-political system would I wish to exist if I did not know to what position I would be born - poor, health, rich, unhealthy, smart, ugly, dumb, beautiful... etc?" I take everybody's preferences into account. I would wish to have a political system that makes the worst-off the best-off they could have been. I would not want to risk being born poor into a system that punishes the poor. So I support whatever makes the worst-off the best-off. That, empirically/historically-speaking, is a mixed economy, which has a flourishing private sector, a healthcare system free-at-the-point-of-use, and a liberal economic system. I don't think it would justify using state force to make the wealthiest pay taxes for those who disrupt that sort of society. Our obligation to obey our society's rules is rooted in the fact that they're fair.
As is clear from my writings, I disagree with the premise that system Y (our system) is "arranged for the benefit of" group B (i.e. the richer group).
The reason the poorest should obey the rules of system Y is because it's the only system which will protect their asses from harm, worse poverty and tyranny. If you kill the City of London, for instance, then you do damage to worst-off in society, because it's their taxes that have been funding a lot of benefits that allow the less capable to continue their more dignified 21st century lifestyles.
The poorest in society have just as much opportunity and freedoms to earn a living like me. Sure, some people aren't so lucky, or perhaps might not be very talented in some respects; but my questions deals with those who don't try, for which, I feel, there is no excuse. There is free education at the point of use, free healthcare at the point of use, and the wealthiest and most talented are compelled (at threat of state violence) to use the fruits of their talents (earnings) help the less fortunate through taxes. Hardly a system geared too much towards "the rich" (which is a flexible financial state, not a static group of people).
"There's no moral obligation on any party to obey the rules of a system which is not theirs and which primarily benefits some other party."
I've already tackled this. No further work deemed necessary, except to say that you'd find it difficult to justify obedience to any set of rules if that is your moral condition. It is "their" system (in some sense), and the system is designed to benefit the worst-off (which capitalist societies have historically done). A lot of poverty is caused by lack of ability or will; rather than opportunity. It is usually lack of will caused by inherited family attitudes (or ability) and the choice to hang out with certain groups of people with similar mentalities. We have access to the internet, to free education, to free healthcare, and such like. There are poorer people who have had a lot less help in life than me, but self-pity is a corrosive poison on young minds which will eat away at their dreams and aspirations.