At the V-Consp, David Post notes a curious case. AFDI wants to advertise on the back of NYC MTA buses. And the content isn't what you might call "friendly." Indeed, it's exactly the sort of inflammatory rhetoric the First Amendment needs to protect (recall that inoffensive, anodyne speech is the mild farina of euvoluntary rhetoric, requiring no Constitutional protections at all, since no one has cause to object).
Private entities have, with the exception of unprotected classes of speech, full editorial control over the content that appears on their property. If MTA weren't owned by the city, they could tell the AFDI or the Muslim Brotherhood or PETA or the FBI, or whatever other hate group* du jour to roll it up, bundle it with a quick burning fuse, stick it where the sun don't shine, and light it on fire.
Commercial advertising may be crass, uncouth, or unlovely at times. But it's not even remotely as ugly as political speech. Private transit services, whatever their perceived flaws, have little incentive to run advertising that threatens to alienate riders. Residual ownership internalizes the costs of venomous rhetoric. Perhaps that might be an overlooked check mark in favor of privatizing public transit.
See Munger for more on the political economy of public transit (links and other information below the 'play' button)
*please recall that "hate group" is a relative term. I cast no aspersions here, merely note that nearly any vocal advocacy group with a bent towards adversarial rhetoric can plausibly be considered a "hate group" to someone.