I don't think it's a bug. even in the Beta they do the same thing but think about it the Terrens are diplomats. they would much prefer to talk with you to stop you from attacking them than actually attacking you themselves. I think it's great that they don't build weapons at all but I think that if someone attacks them they should reaserch weapons. sometimes I give them space weapons just so they won't get creamed by a powerful civilization and change the balance of power in the galaxy. But it is'nt just the terrens if you pay attention the Drath, and the Altarians all reasearch "space Weapons" much much slower than any other civilization. 

Were you seeing ships with defense and no weapons in fleets or just parked around planets?

I reported as a bug during the ToA Beta seeing planetary defenders with defenses and no weapons.  It might make sense to buy and launch a very cheap ship consisting of only a tiny hull to block unarmed troop transports lacking an armed escort; however, adding lots of defense only increases the cost and won't stop an attack ship with a basic weapon (even a ship with high defense will during a battle eventually roll a low defense and take damage).  This behavior was not limited to the Terrans, as I seem to recall the Drath do this as well.  I might be all AIs or all AIs based on the Terrans.

Parked in orbit, all defense, no offense. 

In this second game I took about 3/4 of thier planets before they built a single ship with a weapon.  It seemed they were stuck in "constructor" mode.  They had hordes of them and would try to repeatedly rebuild a starbase while my battleships just used them for target practice.  

What's really sad is this was at least 2 yrs into the game, and they had a lot of planets and a good tech run.  I suppose I should check the error log to see what they had researched and when. 

So the question remains, how often does this happen?