Using the constrained inference net with the unconstrained decoder will bias the decoder to use the anchor words in the aligned sentences – the model is not required to do this, but variational learning will pull the inference network and true model posterior towards each other (i.e. the ELBO objective pressures them to agree).
if the inference net is constrained, but the decoder is not, learning will try to find a weakly constrained decoder to match the approximate posterior.