They weren't, really. "Shunned" implies a communal attempt to bar them in some way, which absolutely did not happen. That said, the immigration experience of Holocaust survivor Jews was definitely complex.
About two thirds of Holocaust survivors went to Mandatory Palestine/Israel*, but of the remainder the majority went to the United States (though US immigration quotas didn't make it easy!). When they arrived, there were a number of social services organizations run by the American Jewish community designed to help them integrate- whether existing organizations like HIAS, the JDC (Joint), and JLC or new purpose-created organizations like United Services for New Americans (USNA) and New York Association of New Americans (NYANA). There were also more informal organizations, such as landsmanshaften (self-organized fellowships of emigrants from particular European Jewish communities), religious charity networks, and social groups/organizations organized around political interests like Zionism and socialism, which stepped in as well. These organizations were able to help with immediate needs such as immigration, housing, jobs, education, religious services, and more, and provided a massive service that allowed survivors to become integrated Americans.
That said, the care provided by these organizations, particularly the larger ones was both limited (monetary grants were only given for the first year) and often did not take into account the psychological (and physical) trauma of the Holocaust and the effect that it had on survivors, particularly adult survivors. In one woman's case, after she reported depression and inability to work and sleep, her case worker assumed the issue was menopause, though only a few years earlier her husband and daughter had been murdered. The connections between the survivors' experiences and their postwar conditions (which could include physical conditions like tuberculosis and infertility and psychological conditions like PTSD) were too often neglected. In addition, some survivors, particularly women, also found these organizations to encourage them to be mothers first and foremost, which could often be difficult for women who in many cases had been well educated in Europe. Social workers often encouraged survivors to put their pasts behind them and focus on the future- not necessarily what they needed to hear.
Socially, many survivors did integrated into existing American Jewish communities, but others found it more difficult. This wasn't due to a coordinated shunning of survivors by any stretch of the imagination- instead, it was due to a massive gap in understanding exacerbated by the 1923 immigration law changes. Because of these changes, there had been relatively few Jewish immigrants to the US over the previous 25 years (after a previous massive influx), which had allowed the existing Jewish community to generally integrate into US society and ended up creating this gulf. Survivors often found that they were not understood by their fellow Jews, who by now were often second- or even third- generation Americans who could not relate to these new immigrants (who often reminded them, negatively, of their own immigrant pasts). While many Jews really did make a concerted effort to include and integrate the survivors joining their communities, even in those cases the survivors still felt alienated and misunderstood by people who had been living lives of relative luxury over the past few years while their own lives had been destroyed. As a result, many survivors ended up somewhat forming their own sub-communities and organizations where they could socialize with people who understood them.
To sum up- there was no shunning. But the American Jewish community could definitely have been better to its survivors.
*Israel had its own issues, which I talk about more here