Leaving lock on the parking rack - is this rude?
Whether it's rude is a matter of local custom. Whether you're offended by it depends on your flexibility and sensitivity

Often, we cyclists can become a little over sensitive, after having to defend ourselves from car doors, pedestrians, cars, trucks, laws that don't take us into account, police who have a ticket quota, and even other cyclists.

Relax.

The person who left it there is just trying to save themselves the trouble of carrying it every day.

And yes, you can still park your bike there. Of course they may think you're rude then, so I'd only park there if it was the last position available.