Apparently you cannot back the police or in this case the security forces with the Thin Blue Line flag because it is now considered racist:
The Air Force acknowledged that the display by security forces airmen at Ramstein Air Base of a “thin blue line” flag violated Defense Department policy on what types of flags can be flown on U.S. bases.
The 86th Airlift Wing posted and later removed photos on its Facebook page of airmen in uniform carrying a black-and-white American flag with a blue horizontal stripe during a 24-hour rucksack march as part of National Police Week.
The photos drew immediate condemnation by some in the community who questioned the display on a U.S. military base of a flag that has become a polarizing symbol in a nation grappling with police violence and racism. The “thin blue line” flag is used to show support for law enforcement, but it’s also been used by white supremacist groups and flown at far-right rallies.
Stars & Stripes
You can read more at the link, but based on this logic supporting firefighters with the thin red line flag should be banned as well. Taking this to its most extreme would be banning the U.S. flag since they are carried by extremist groups as well.