Eritrea supports the Islamic factions in Somalia in a proxy war against Ethiopia.
Back when the Islamic Courts were riding high in Somalia (before the Ethiopian intervention), there were a number of reports of Russian made charter flights moving "special shipments" (read: probably weaponry) into Mogadishu (believe it was).

I wouldn't be at all surprised if this step is one of those actions where the US government wants to make sure that those type of shipments don't reoccur, and they are trying to "persuade" the air charter companies that such dealings for Eritrea (shipping weapons) wouldn't be such a good idea in the future.

Probably one or more of those companies told them that there's no violation involved, so why should they pass on business. Well, the US government looks to want to give them a legal reason not to do this type of business.

Also, doesn't designating a specific nation as a "terrorist" nation give the US Treasury carte blanc to start designating businesses/governments as being engaged in terrorist/supporting activities, and therefore can limit their access to financial credit markets?

That's a hammer.