Is that normal? I thought the NCR and BoS didn't like each other.

I was just walking along and had to double-take because I thought he was BoS at first.

They fought at Helios one, they use the armor as protection. BOS only acquired it as old world technology so anyone could have it if you got your hands on it.

Oh ok I see, that makes more sense.

Ya at the dam you will also see some with it on.

Read all about it http://fallout.wikia.com/wiki/NCR_salvaged_power_armor. It is not really power armor, more a stripped version so soldiers can wear it without power armor training. Also the EMP-weapon bonus does not work against this armor.