I don't view property rights as absolute, but rather I view limited property rights as a mechanism to create incentives for a nation's people to become productive. In other words, property rights exist not to protect the rights of the individual, but rather to serve society. The mistake many people make is to view them as serving the individual, and then start to mistakenly believe that they are somehow absolute. No rights are absolute. They all only exist as long as they serve the purpose of increasing the good in a free society.
Since I don't view taxation as theft, I am free to recommend that tax system which best serves a nation. I don't really care about fairness, since there are so many ways to look at such a notion. The only thing that really matters is that tax system which maximizes both prosperity and spread of the nation's wealth. Sometimes these are in conflict and it is up to society to determine the correct balance. However, just the maximization of prosperity itself is best served by a progressive tax system.
Any time society attempts a system of property rights and combines it with any tax system that gives rise to a pronounced concentration of a nation's wealth in the hands of a relatively few, this leads to economic malaise, corruption and poverty for the vast majority of the population. I like my society set up with a set of fairly strong and extensive property rights to foster a free market system, combined with wise regulation of those markets, and finally combined with a progressive tax system to offset the tendency of wealth to accumulate at the top.
This kind of system is a whole package, and it is "Fair" because it works. If a flat tax was what "worked", I would be for that (in emerging economies, there is evidence that a flat tax is best). Whatever works.