Why are they not citizens? Because you say so? Because the US government says so? Bending the law to suit your agenda is not how it works and once you do it the law is just some letters in some book. You either uphold the law in it's entirety or you don't. There is no middle ground.
As to the question of them being guilty....the fact that the US has lost 75% of the cases against people they are illegally detaining should be a clear indication to you a lot of them were indeed picked up off the street for no reason.
... What are you talking about? They are, by definition, not citizens... I don't really understand the confusion on this one, and we're not even talking birthright citizenship here. You're right, though. On this one, there is no middle ground: Foreign nationals, born in a foreign country, not living in the US, and not having citizenship or even residency in the US clearly makes them "not citizens."
And simply because a percentage of these prisoners were released says absolutely nothing about their status as "guilty or innocent," it simply makes a statement towards the concept that they had been improperly detained. And simply because someone has been improperly detained has nothing to do with the reasons why they were apprehended in the first place. There are several cases where suspicion that leads to detention does not ultimately lead to conviction. These individuals that have been released thus far may, or may not be innocent of crimes, but simply because there is no evidence to the contrary has no bearing on whether or not they are actually innocent or not.
Simply because a person has been "unlawfully detained," does not speak to innocence or guilt, it simply means that the evidence provided by the prosecution was not compelling enough to warrant further detention. Its all up to interpretation by the specific judge ruling over each specific case. And in a percentage of these cases, the judges were not compelled enough by the arguments made by the prosecution to further detain the individuals. It doesn't mean that they were/are innocent, it just means that there wasn't enough evidence to find that they were guilty. Google OJ Simpson, Sam Sheppard, or the murder of Jessica Lall. Just because a person is found "not guilty," doesn't at all mean they are "innocent."