These were not REALLY illegal immigrants. The EAC common market which Tanzania signed and ratified gives every East African the RIGHT of residence in ALL east African countries subject to the individual EA laws on the basis of their national interest. Now some interpret this to mean the East African countries can do whatever they want. NO this is not the case. The additional clause is mean't as an escape clause to be RARELY used unless you REALLY need to. In effect ALL East African countries are breaking the spirit if not the letter of the treaty. Technically you could take them ALL to the East African court of justice but you might lose due to the escape clause.
Here the problem is Tanzania as well as other countries EA countries are using ILLEGAL laws to call East Africans illegal immigrants.