Authorities do have the authority to cover up graffiti, sure, and they have the right to impose the will of the majority (for a plain beige environment) and to overcome the desire of the disenfranchised lawless minority for self expression.
When I don't like what they expressing (if it's not aesthetically pleasant to my own subjective taste, when it's crude, when it expresses allegiance to a group of people who advocate violence, when it expresses a political opinion with which I disagree), I am all for beige.
When I like what's being expressed (simple expression of individuality, or where it shows technical skills in executing a pleasant or interesting artistic expression), then I lean in favor of the lawless artists.
I do appreciate art (as I define it), wherever I find it, and I don't like to see it disappear. I recognize that one thing that makes good street art valuable is that it is ephemeral. And in the end, I also appreciate the tension between the lawful advocates of beige and the unlawful energy of chaotic expression. Both forces ought to exist, and both forces do good things.