The whole 'don't kill builders' thing gets on my nerves for two reasons:
1) The trench warfare especially is very similar to ww1, where a ton of people got massacred for no apparent reason. I see no reason why a repairman could walk out into no-man's-land and start making barbed wire without being aimed at by the enemy
2) When the idea is enforced, it gives the ex-victim the opportunity to gain an advantage over the opposition. By declaring 'I'm building' in most servers you can build an entire fort and anyone who attacks you is kicked/banned for no reason other than 'you killed a builder'. I mean seriously...
Use non-combatant player if you want to do what you want, but it's war. Live with it.