By choosing to lobby for less taxation/redistribution and taking advantage of the benefits they lobby against they are folding on their principles.
And yes, the same for those who argue against tax cuts. They give up their principles by not sticking to them when they are inconvenient.
But there's also a practical concern: it's not possible to keep track of all counterfactuals / hypotheticals when it comes to tax/welfare laws. In Ayn Rand's case, she couldn't afford to live without social security in her later years (if the reports I read are to be believed). We frequently don't have the means to be consistent in our principled stands.
It seems like your 'practical concern' results in a ratchet against those in favor of simpler systems (of taxation, regulation, and welfare). The advocates of complexity can always renege, while the advocates for simplicity must always suffer.
By that logic, anyone who unsuccessfully campaigns against tax cuts should continue to pay the previous (higher) tax rate.