The question is how does one go about taxing imputed rent? It is not easy. One could start by imposing an ad valorem tax on property values (such as a local property tax), but that doesn't tax imputed rent per se, because it does not take into account expected inflation (if one person expects her house to go up in value, and another does not, the rent the first person pays is lower than the second). Alternatively, one could find comparables in the rental market and attribute rents found there to the owner market. But owner and rental markets are so segmented that this would be difficult to do.
This has implications for fairness; if we don't know what we are taxing, it is hard to know how much to tax it.