I am pretty sure they run such sales periodically - free upgrades. The problem is you have to read the fine print. Many upgrades are just from the same cabin but on a higher deck.
I think I just saw an NCL sale that said "free upgrade, oceanview to balcony" - but this is the Holland America thread.
otherwise they mostly sell them, because they can. When they have exhausted that they give them away, but usually at the last minute.
I think that some people who get an upgrade early one only do because they would have dropped the price to the point where what you paid would have gotten you a balcony cabin anyway.