  The CP asymmetry in $\tau\to K_S\pi\nu_\tau$, as measured by the BaBar
collaboration, differs from the Standard Model prediction by $2.8\sigma$. Most
non-standard interactions do not allow for the required strong phase needed to
produce a non-vanishing CP asymmetry, leaving only new tensor interactions as a
possible mechanism. We demonstrate that, contrary to previous assumptions in
the literature, the crucial interference between vector and tensor phases is
suppressed by at least two orders of magnitude due to Watson's
final-state-interaction theorem. Furthermore, we find that the strength of the
relevant CP-violating tensor interaction is strongly constrained by bounds from
the neutron electric dipole moment and $D$-$\bar{D}$ mixing. These observations
together imply that it is extremely difficult to explain the current $\tau\to
K_S\pi\nu_\tau$ measurement in terms of physics beyond the Standard Model
originating in the ultraviolet.
