It doesn't seem like they've been fired, at least not yet, just sent back home.
Most likely it'll just be a slap on the wrist for the agents and they may not be sent out of country again - Secret Service agents aren't that easy to replace given they have specialized training - at least as most of the details of the case stay swept under the rug.
I don't have a problem with prostitution so long as it involves consenting adults, but I can see why this was a potential issue diplomatically. The Secret Service agents were in Columbia to do their job, not to be on vacation, so if they cause a scandal it wouldn't be good for a diplomatic mission. It doesn't particularly matter from a diplomatic standpoint if they were on duty at the time or not. Also, the misconduct in question could involve more than just being with prostitutes, but could be some other trouble they caused while being with prostitutes, which would just make said misconduct all the more embarrassing. We're lacking on details, so it's hard to say.
From my thorough research (cursory Wikipedia glance) on the subject, it seems that A. (indoor) prostitution is legal in Colombia, but B. Cartagena in particular suffers from a high number of people in sex slavery and child prostitutes.
In my view, as long as "B" doesn't come into play, "A" means that what they did, while stupid, shouldn't get them fired.
In a poor country like Colombia prostitution almost always include an element of exploitation.
