(Preface: I have all the game/stuff packs.) Since updating to the latest patch, I've noticed that if I have either of these objects (observatory or woodworking table) placed on the lot, my sim will stop whatever they're doing and start using that instead. Or they'll do what I tell them to (eg, read a book, cook a meal, etc), sometimes stopping mid-action, sometimes completing it out, and then decide to go use the observatory or craft something, even if they didn't have a logic or handiness skill beforehand. If I cancel the observatory/woodworking action and tell them to do something else, it'll just pop back up again. They seem obsessed with these objects so much so that I can't even have them on the lot unless I want them to be using them.

I do use a lot of CC as well as some no-auto mods (like no auto phone interactions, no auto grab a drink, etc), but none that I would think would affect these two objects... I tried CC observatories and woodworking tables, and the same thing happens with those. I'm just wondering if anybody else has this problem or if it's just being caused by some conflicting mods I'm using… I'd hate to waste time debugging my mod folder if it's a common problem. I tried running the game with no cc/mods, but then my sims just seem obsessed with using their phones or else doing pushups/situps, which is not really ideal either...

It came with City Living. At the art-district there is a workbench in the art-hall that can be used by everybody, and during the geek-com it's the same with the observatory - autonomous using.
And - as you said - VERY MUCH autonomous using, it was driving me mad!!
After finding out that many have this problem and after trying this and that, I found this solution for me: I put a fence (or walls) around the workbench/the observatory, with a door/gate. And I lock this door either for every Sim, or only for the ones who don't need logic or skill. Only the best are allowed to go through the door! Or nobody if they use it too much.
I hope this helps a little.