Owning a milk crate is not illegal.
Possessing stolen property is illegal.
If you bought the milk crate from a milk crate supplier, then it’s perfectly legal. If it is stolen from a dairy or other company, then it is illegal.
My big annoyance is that it is really, really hard to buy the good milk crates legally. I know they would be expensive, but I don’t want the cheap, crappy ones, I want the good ones that are sturdy…