Why are empty pylons weighed down? (airplane-pictures.net) Airbus A330 no engine option.

Why are empty pylons weighed down?


The plausible answer is to maintain the correct center of gravity. But why not just load ballast pallets in the forward cargo compartment, like those used on tail heavy planes on ferry flights?
On airliners.net there is a theory (plus debate) that states another reason is to relieve the load on the wingbox, which I would imagine would also decompress the landing gear (I don't see how).
Lastly, I remember on a TV documentary it was stated that the reason is so the wing would not deform upwards (again I don't see how).
Your reason 1 is correct. Without the ballast the aircraft would become a taildragger.

Why not pallets? This would produce the same center of gravity location, but a different mass distribution. Clearing this configuration even for being loaded and pushed around is more effort than simply placing the ballast where the mass of the engines would go. Now all clearances are valid and the aircraft can be handled much as one with the engines in place.

Next, engines are only installed shortly before delivery or first flight. The cost of capital is too high to install them any sooner.

(airbus.com) Example of clearances that would be affected by non-standard mass distribution.