Because the trustee of the domain (which one would assume would be an organization within the country) has administrative rights for the domain. And that trustee can decide to require local presence to use that TLD, which might leave this particular service offline, at least for a time.
It does however tend to color your judgement about how professional and reliable this service is. If they risked their future on the stability of the Somalia domain registry - just so they could have a quirky domain name.
Calling yourself slashdot is funny if you are a geek site - worrying if you are my pension fund.
The nameservers for the .so domain are not physically located in Somalia: http://www.zonecut.net/dnsgeo/SO..html