There are laws, and social mores - but those certainly don't stop people from hacking road signs or removing traffic signals.(1,2) Moreover, unlike spikes on the road, a tampered-with indicator might not be immediately obvious, even in its effects.
Anyway, I finally had a chance to read through the patent; it's enough of a concern that they specifically address it:
"In some embodiments, the data provided by the reference indicator 250 may be encoded or cryptographically signed for security purposes. By providing encoded or cryptographically signed data, the ability for a third party to disrupt the autonomous vehicles may be mitigated."
