I don't think it is a matter of whether you should or shouldn't be
redirecting from a model; that decision is entirely up to you based on
how you define your development pattern.
For me, the definition of the MVC pattern is very logical and effective.
If you want to follow that definition, then it would be illogical to do
a redirection from within a model because models 'model' data in a
domain specific representation. Controllers primarily govern the
interaction between the user and the application, a redirect would seem
well suited to that domain, such as, moving the user from this place to
that place based on a satisfied or unsatisfied logical constraint (is
user logged in, or not?).
Ultimately, the decision is up to you; a theoretical understanding of
design patterns would help. It may serve well to research some of the
opinions about why design patterns are useless (vs the pro-pattern
opinions), an informed and logical development cycle is much more
effective than one without focus or decisive choice.
