Scanners have gotten better recently, meaning they can tell the difference between water and explosives, but these restrictions weren’t pulled out of thin air like many others.
https://en.m.wikipedia.org/wiki/Security_repercussions_due_to_the_2006_transatlantic_aircraft_plot
There was a thwarted terrorist plot to use drink bottles to hold explosives.
Others have given the rap beef context, but nobody has mentioned why a blood would have been on stage or why that was significant. At the end of the concert, Kendrick got a bunch of people from a bunch of LA gangs on stage and made a big show of unity.
TMZ article - mainly as a source, the article itself isn’t fantastic