Why do employers care if you smoke weed as long as you come to work sober.?

So weed being virtually harmless I don't see the big deal, if it was meth and herionthen yeah that bad but weed shouldn't matter (THIS IS NOT ABOUT WHAT DRUGS ARE OKAY AND WHAT ARE NOT SO DONT EVEN BRING IT UP)

So today on my first day my employer asked if I'm willing to take a drug test, I said yes, but I do smoke weed so that will come up, he said alright and I took it. It came back positive for weed only, he said since I was honest he will let me stay but when he tests me in a month or so I better be clean.

But I wonder why is it so bad? I use it for anxiety, Im not medically prescribed it but it helps, and yes I do it to just get baked too.

But as long as it's a drug that won't take over your life and ability to function when not on the drug (like withdraws from herion affect you when you don't use) why does it matter as long as you come to work sober.