Left and right are descriptive terms commonly applied to most political diametrics. That includes economics, social politics, etc. That's how they've been understood throughout modern political history, since they came into being to describe French monarchists and revolutionaries.
In terms of taxation, pretty much nobody would dispute that more "progressive" systems-- which place more of the tax burden on higher earners and corporation tax, and/or act in a redistributive way-- are associated with the left. The tax system placed a larger proportion of the burden of tax on the higher earners under Truman, Eisenhower, and Kennedy than it did under Obama.
This is in part, of course, because of their different starting points. Obama inherited a very different Overton Window with regards to tax. But if you're going to be making comparisons between Presidents in vastly different times anyway, then you cannot simply ignore tax systems have been vastly more progressive in the past.
So you're basing it on an assumption.
An assumption Sanders has been quite explicit in not making (see his statement on the matter), and using that assumption to justify doing the opposite of what Sanders has encouraged voters to do.
...And you believe this is the most effective way of bringing about the kind of politics Sanders represents.
With voters like these, who needs opponents?