I see no problem considering you are in a urban area.

Also even in the wild for real one spider out will not make a difference (even if everyone who wanted a spider did that), the problem is when people capture spiders comercially, then the number captured will be bigger, that said I think it's ok to get one yourself, but not to buy a wild caught.

I don't see what problems you say regarding keeping a widow as pet, a lot of people keep them just ok, make sure the enclosure is escape-safe, and use a container upside down so the lid stays at the bottom and you wont disrupt it's web when you open it (DVD tubes works nicely). Also hotglue something on the walls to encourage web anchoring, like sticks and roots.