I have tried to find good reporting on this issue since it is scarce in the American press. In the American news stories that I have read, there barely a mention of the people who have taken hostages or why they have done what they did, rather they are only referred to as "militants." I have pasted a couple of articles below, and the BBC at least makes an effort to explain why The Movement for the Emancipation of the Niger Delta (MEND) has taken hostages and disrupted oil production in Nigeria. As one BBC article reports:
The BBC's Abdullahi Kaura Abubakar managed to meet one of the group's leaders, who used the alias Major-General Godswill Tamuno...Mr Tamuno told our reporter that Mend was fighting for "total control" of the Niger Delta's oil wealth, saying local people had not gained from the riches under the ground and the region's creeks and swamps.
He said the Delta had been exploited for the benefit of other parts of Nigeria and foreign companies and ordered all oil companies and Nigerians whose roots lie elsewhere to leave the region.
Here are various articles on the subject, see for yourself the difference in reporting:
Nigeria Militants Won't Release Hostages from the Washington Post
Nigeria's shadowy oil rebels from BBC News
Nigerian militants claim attacks from BBC News