Previous Mishnayot have taught that a person whose testimony allows a woman to marry (e.g. a person who brings a writ of divorce from a far-away land, someone who testifies that the husband had died or been killed, or even a Torah scholar who refuses to annul vows that the wife had taken against her husband, forcing them to divorce) cannot, himself, marry her; we suspect in such cases that he has an ulterior motive for his actions – that he desired her for himself. The Mishna on our daf teaches that if such a witness was married at the time that he testified, if his wife dies, he would be permitted to marry her, since at the time of his testimony his situation as a married man makes him “above suspicion.”
It is interesting to note that in the time of the Mishna the Biblical law permitting a man to marry more than one wife was still in effect (the practice was not discontinued until well after the Talmudic age, when Rabbeinu Gershom established limitations on such marriages. Even so, Sefardic communities did not accept this limitation until the modern age), so theoretically the witness could have had an interest in marrying this woman. Nevertheless, such marriages were relatively rare, and the Mishna did not consider such an unlikely concern to be reason to establish a prohibition against marrying her. In fact, the Nimukei Yosef quotes a statement from the Talmud Yerushalmi (which is not found in our texts) that teaches that if there is reason to suspect him – if his wife is sick, for example, then we would not allow him to marry her.
The Rivan explains simply, that since the witness did not marry her right away, but waited until after his wife’s death to do so, all suspicions are removed. This concept is expressed by the Yerushalmi in the words ein adam matzui lahto le-ahar zman – we do not suspect a person of sinning if he will only benefit after a significant amount of time will pass.