As TheOtherLeadingBrand commented, "ethics" seems like a somewhat odd word to use. I certainly don't believe that a responsible adult keeping a large constrictor in an appropriate enclosure, caring for it properly and not showing it off in an irresponsible manner is "unethical." (Though there are, to be sure, many, many people who do believe this.) Maybe the word you were looking for was "logistics?"

As far as safety goes, my feeling is that there are certainly ways to minimize the risk, most of which have already been mentioned -- don't smell like food, know how to "read" the body language of snakes, etc.. I also prefer to work with animals that are known to be of calmer disposition, rather than disagreeable monsters -- but, there are some who are fine with the latter, even when it comes to giants.

What I think is very important is to realize what an incredibly powerful, potentially dangerous animal one is working with when interacting with giants -- and I think this is something that giant keepers may sometimes forget, especially when working with big, seemingly tame "slug-snakes" like Cookie here (my Burm, pictured below). Before I purchased her, the seller sent me pictures of his (very young) son interacting with the snake. While this seeming display of "tameness" was one of the things that made me want to go have a look and eventually buy her, I think it could be argued that this was a fairly irresponsible thing to do to allow a child to interact with such an animal, and that it could even be considered to border on "unethical." (Similar to how, the other day, I saw the three or four year old daughter of a horse trainer riding around bareback on the back of a horse at a horse show ...)

I do believe that anyone wanting to work with these animals (giant snakes, or horses, or certain types of cattle, or alligators, or ... ) needs to realize the potential for a life-threatening situation to occur in an instant, plan accordingly, and accept that, to some degree, you are always at least somewhat relying on the animal doing what you want it to do. There's always an element of chance and perhaps misplaced trust, though I trust my Burm and the horses that I sometimes with with more than I do the drivers in this area, and I drive on the road every darn day ... So I guess it's all a matter of whether you're willing to take a risk solely for the enjoyment of the animal.

Boy was that a rambling post! ... Sorry for that.