I agree with you. If it's a junk car and unsightly, they can cite you for that separately. Citing you for expired registration is just a $$$$ play, that I don't think that should extend onto private property, so long as the vehicle is not causing a nuisance, or is unsightly, etc.
What's really being discussed here is MCCD wants the ability to run around collecting money for expired tags on private property and apparently finds it too much of a hassle to leave the air conditioned comfort of their city-owned vehicles to check the tag. So instead we're going to pass a law governing how people are allowed to park their cars on private property.