For my deduction, they either put up their signs for political or superstitious reasons.
Political coz they want to look good, on the "right side" and they may lose sponsors if they don't put up the sign. Superstitious coz they want to maintain "spirit-free", or feel that shugdenpas will bring them bad luck.
Religious hypocrisy at its apex.
Anyone who seeks a spiritual home should be naturally turned off by those signs. How can you shun away those who seek to learn the Dharma, regardless of who they choose to pray to?
How can you welcome Christians, Hindus, Jews, Muslims etc, and ostracise a fellow Dharma brother/sister? If they truly believe in reincarnation, cause and effect, they wouldn't put up such negative influencing signs. I won't be surprised if those who reinforce the rule are reborn as lepers who will have no option but to be segregated from society.