Whilst you consider it informal, in many parts of the world it is quite common.
For example, sports stars are often referred to by their surnames. In military or police settings, surnames are often used (e.g. Private Martin, or just Martin). In many old-fashioned private schools, surnames are used.
There are a variety of possible reasons for this, but usually it relates to either:
- Depersonalising a situation
- Demonstrating authority
- Separating people by using surnames rather than more-common first names
- A combination of the above.
In a sports setting, it's simply far easier to refer to people by surnames, as they're more distinctive. Now, for particularly famous people this might not be the case (think Cristiano, or Kobe, or Lebron. But for the majority of others, it's far more distinctive to use a surname.
In a school setting (particularly traditional private schools), it both personalises and separates out people. Rather than having a class with five "James"s, teachers used surnames to distinguish between them. It also created a more authoritative atmosphere which was used to help control classrooms.
Finally, in your military/police setting it's likely to both depersonalise a situation and demonstrate authority. It shows that everything is not immediately "chummy", but that authority and respect of authority is key.
If you search online, you'll find many, many shows following this pattern.