True, many people raise the "United States persons" issue.
First, a "United States person" is a citizen or permanent resident only. Not someone on a travel or work or student visa. It's not a simple presence test.
Second, it's arguable (as the NY Sun notes) that even naturalized citizens or permanent residents communicating with terrorists overseas have a de facto problem with their immigration status (and thus as "United States persons" under the law), e.g., you cannot have legal standing as a "United States person" if you lied about terrorist associations in U.S. immigration documents.
Third, what is "substantial likelihood"? 1 percent? 5 percent? 20 percent? It's arguable that if only 1 percent of the surveillance targets turned out to be legitimate "United States persons", that this falls underneath the threshhold of "substantial likelihood".
Note that the MSM has not treated us with any specifics whatsoever about the number of "United States persons" targeted by this statute (but of course, they haven't mentioned the statute itself, either).