First he was detained due to information from Canada.

Second there is other information that the US thinks is sufficient to keep him on the no fly list. They are attempting to share it with Canada as soon as they respond.

And he was not an innocent citizen as you keep claiming.

The trouble started in Tunisia before he departed for Canada:
UNITED STATES DISTRICT COURT
EASTERN DISTRICT OF NEW YORK
MAHER ARAR,
Plaintiff

"...in an investigation by "Project A-O Canada," a Canadian team
investigating terrorist suspects in Ottawa. According to the
letter, Arar was contacted by an investigator with Project A-O
Canada on January 22, 2002, during which time he was in Tunisia.
On January 25, Arar told the investigator "he could perhaps be
available" on Monday, July 28 for an interview. See LaHood
Letter at 2. Later that day, however, Arar's attorney "contacted
the investigator to advise him that there would need to be
parameters for the interview." Id. at 2-3. The attorney
"requested that the interview take place in his office and that
Mr. Arar's statement not be used in proceedings as a substitution
for his actual testimony; clearly the information gathered could
be used for the investigation, and nothing would preclude calling
Mr. Arar to testify." Id. at 3. As a result of these conditions
– which were shared with U.S. officials – and because the
investigation wound up focusing on other areas, Arar was never
contacted again for an interview. See id. The LaHood letter
claims that, in light of plaintiff's decision to exercise his
constitutional right to remain silent, which was known to U.S.
officials, Arar's interrogation within the United States took
place in disregard of Arar and his attorney's request.
To some extent, the contents of the LaHood letter undermine
plaintiff's claim, "on information and belief," that there has
never been, nor is there now, any reasonable suspicion that he
was involved in such activity. Although the account of what
occurred in the Canadian investigation could not give rise to an
adverse inference in a criminal prosecution, the change in Arar's
posture would certainly justify at least some suspicion (and
perhaps reasonable suspicion) on the part of U.S. officials
during their investigation about Arar's activities...."

Why would he need a lawyer in Tunisia? Perhaps if he had cooperated fully with the Canadian investigator in Tunisia the whole thing would never have happened.