This seems to assume that the ruling class is unified and has no internal conflicts. It seems obvious to me that this is far from true. Of course, I don't know what your definition of "ruling class" is. I would probaby define it, for my own uses, broadly, as the upper 1 percent of all the major proffessions, plus the wealthiest 5% of the public (I know, substantial overlap) and all large corporations. Whether to include small business is a big question. For the most part, I wouldn't. Across the street from my apartment is a pizzeria, and as I look at it, I have a hard time thinking of that Italian family who runs it as members of the ruling class. Still, my definition includes the upper tier of lawyers, doctors, computer programmers, business managers, engineers, architects, screenwriters, etc, plus the interests of the corporations themselves. That a computer programmer might sometimes have interests that are at odds with, say, General Motors seems obvious enough, but even within the corporate sector, that Microsoft might have interests that are at odds with, say, Apple, also seems quite obvious to me. Therefore the ruling class has perfectly good reasons for resisting taxation: when they keep the money in their own hands, they can be sure the money will be spent to further their own ends, but when the government takes and spends the money, the ruling class can not be sure that it will be used in a way that helps them at all. Again, for instance, a portion of the taxes that General Electric pays goes to fund the EPA. Why should they rejoice in that?
More so, I think its reasonable to point out that industrial life in the modern democracies has aspects that are fundamentally middle class and that no one escapes from. All business managers, no matter how wealthy, travel in cars and thus have an interest in car safety; even the lawyer who defends General Motors in class action suits probably drives home hoping his or her car has the latest and best safety features. If anything, such a lawyer might be more aware than most of which models to avoid. Not that he or she would ever say so out loud.