Cops protect private property, so if the zeitgeist shifts so that fundamentalists with their giant trump crosses are bringing down property values, then yeah, the cops will come hassle them.
The issue here is that they think "The Bible is the word of God and i believe what it says" and "I hate gay people and wish they would all die" mean the same thing.
36 “Teacher, which is the greatest commandment in the Law?”
37 Jesus replied: “‘Love the Lord your God with all your heart and with all your soul and with all your mind.’[a] 38 This is the first and greatest commandment. 39 And the second is like it: ‘Love your neighbor as yourself.’[b] 40 All the Law and the Prophets hang on these two commandments.”
When the bible is the one saying gay people should die, it kinda does.
The saving grace is that most people who say that don't actually mean it, as they either haven't really read it, or have made enough mental gymnastics to ignore the stuff they don't agree with.
This is a real possibility; any view can become illegal if we do not value freedom of speech. There are examples all over the world and throughout time.
The United States is not special, it can happen here.