"Even more to the point, why didn't they just knock on the front door and ask him what the plants were? Why the SWAT approach?"
It makes me sad that it took this long for someone to point this out. I for one am sick and tired of the "authorities" choosing the smash in the door with guns drawn as the first option. Here in TN a couple of years ago an elderly man was killed by cops when he tried to defend his home against what he thought were criminals (he was right, actually). turned out they had the wrong address. IMO these thugs relish the chance to play soldier and don't consider other, less invasive options like stakeouts, or a simple knock. I wonder if we can get an honest legislator to introduce a law against these tactics as a first resort?
So am I, but I doubt they smashed in the door in this case. As I understand it, that sort of warrant serving is only legal when the police believe that the occupant of the house could get rid of the evidence if they are given a bit of time. As the full sized plants were outside in the front yard, that doesn't seem to apply here.