'Better to be safe than sorry' is not argument that encourages nor supports a free and democratic society.
It is an argument that would support the banning of any open bus parade in the first place for instance.
It may be a "reasonable" request but it certainly isn't reasonable to detain someone who fails to comply with that request and to confiscate/examine their camera equipment UNLESS there was additional cause to believe the individual was involved in terrorist activities.
Requiring all supporters...
Are the following reasonable requests? - questions which if asked of the right people could equally prevent the scenario you outline.
'What are your bank account details?"
'What is your email password?"
"What web sites have you visited"
. . . . and so on.
The key thing is that they are all...