FREE SPEECH -- A helpful reminder for both landlords and tenants appeared in a recent San Francisco Chronicle, supplied by Project Sentinel: California law doesn't allow the former to censor or penalize the latter based on their display of political signs in their windows.

Attempts to regulate this type of tenant activity should be very
carefully considered. California Civil Code Section 1942.5(c) protects
tenants from retaliatory acts based on their exercise of "any rights
under the law." The "No on Prop. 8" sign you are describing is an act
of free speech and is almost certainly within the scope of Section
1942.5(c).

Since you already allow window signs, it would be difficult for you
to argue that this particular sign is not permissible, unless it
constituted some type of hazard because of its size or placement, or
because of some other specific physical attribute. Deciding which signs
can be displayed based on the content, such as deciding that certain
language is "offensive," means that you are relying on a very
subjective standard, because one person's "offensive" is another
person's "free speech."

The referenced code provision states:

  (c) It is unlawful for a lessor to increase rent, decrease services, cause a lessee to quit 
involuntarily, bring an action to
recover possession, or threaten to do any of those acts, for
the
purpose of retaliating against the lessee because he or she has lawfully organized or
participated in a lessees' association or an
organization advocating lessees' rights or has
lawfully and peaceably
exercised any rights under the law. In an action brought by or
against the lessee pursuant to this subdivision, the lessee shall bear the burden of
producing evidence that the lessor's conduct was,
in fact, retaliatory.