I think peer pressure is an apt description, but they see it as something that unites them, and solidifies them as a group, community, etc. It's a symbolic gesture with no real significance beyond the symbol itself. To the missionary working in some jungle building sanitation systems, digging wells, etc., the tattoo is a trivial reminder, whereas the Christian working to justify their own existence views it as something of merit in itself. The act of getting the tattoos and wearing them is what is important to them.
The thing to note here is that with the former it is no more than a trivial gesture, while the latter is viewing this as validating who they are. It is a blatant contradiction to Christ's own words. The only validation is the fruit produced, and tattoos are not fruit.
Let me give an example. A pastor I used to know was doing work on a roof. There were other people involved and someone flipped the latch on the extension ladder so that when the pastor stepped onto it, it began plummeting with him to the ground. He came down on his right hand which immediately sent that bone between the elbow and the hand out into the dirt. He had a tattoo placed over the scar that said something like "This too shall pass". I don't remember exactly what it was, but I think that might have been it. Anyways, it was a small reminder of what he had been through as he was serving others. it wasn't a badge of honor validating his life as a Christian. That's the difference.