Now that's not only interesting; it is something that sociologists ought to be able to say something about. As it turns out, they have. So, let's take the problem apart for a moment.
First, the implication in emphasizing this seems to be that this makes him somehow different from the rest of us. As it turns out, having fewer and fewer friends is a generic problem that plagues US society. Those of us who don't know this from their immediate experience have surely had a chance to learn about it from Robert D. Putnam's widely celebrated book, Bowling Alone. Here the author claims that the interpersonal density of social life of in the USA is waning. In his words, "we sign fewer petitions, belong to fewer organizations that meet, know our neighbors less, meet with friends less frequently, and even socialize with our families less often." In other words, the alleged Boston bomber may not be all that different from the rest of us in this regard. Most Americans don't have many American friends.
Second, there is a very large and distinguished literature on the social networks of immigrants. Two themes seem relevant to this issue. First, immigrants often experience sociality as it exists in the United Stats difficult to interpret, as it is mis-aligned with their "cultural" understandings. In-depth interviews conducted by Elizabeth Gareis, for instance, suggest suggest that "all informants struggled with cultural differences concerning friendship formation. Most prominently, the different category width of the word 'friend' and the extent of public and private personality layers in the two cultures caused confusion and misunderstandings." In other words, what the accused, now dead Boston bomber might have meant when he was not a bomber yet may have been a reflection on this cross-cultural mis-match. We don't know.
In a context that is often un-decipherable to them, immigrants often tend to rely on co-ethnic networks. Work on migrant informality and the social embeddedness of migrant economic action constitutes the classics of the sociology of migration. Taken together, the ties that bind immigrants to local society are important determinants of the character of the context of reception that determines, in turn, a huge part of the migrant experience, not only in a narrowly economic sense, but also in terms of psychological well-being and emotional stability. All those points seem directly relevant to the Boston case.
Finally, work on the discrimination and segregation to which African Americans are still subjected in the United States, has uncovered, approximately one generation ago, that social networks play a significant part in the preservation and inter-generational transmission of disadvantages. William Julius Wilson has suggested, for instance, that "Social isolation deprives residents of inner-city neighborhoods not only of resources and conventional role models, whose former presence buffered the effects of neighborhood joblessness, but also of the kind of cultural learning from mainstream social networks that facilitates social and economic advancement in modern industrial society." (The extent to which the US is an "industrial society" is somewhat questionable, but that's beside the point, as it stands even more for the post-industrial service economy that the US has evolved.)
It is not possible to talk about sociality, friendship and informal ties in the United States without these insights. And that is exactly what the meanstream media (pun intended) does. I wonder why.