But to give a serious answer… I have lived in apartment complexes where they didn’t like me locking up my bike against any kind of light pole or railing on the property. I suspect it didn’t look good 🤷 (I mean it was a nice bike.)

I feel like the motivation for whoever puts up this kind of sign is most likely not trying to help you ensure your property is safe. Otherwise, they would put up a bike rack instead of a sign.