Is there a case where, you and I, or any member of the public
that crosses into a gazetted zone and refuse to obey the
instructions of the police to step back, are not arrested, and only
known miscreants are arrested?
Would you believe that the PAP gave the police instructions, to
arrest only SDP members who cross the line, and not others?
Would any country say "Oh! they are protesting, so temporarily all
laws do not apply til the protest is over." 
Observe the law, even when protesting. Is that too difficult to
understand?
Emotionaliam contributes to skewed perspective.