They were extras for a film. The photo has become an example of, um, yellow journalism at its best.
Look at the photo: Do the soldiers look tense as if they are about to go into action?
Do you see any weapons?
Would they go into action unarmed?
Now think: would be assembling covert operatives out on a public street in full view of, say, press photographers?
If you dressed up your covert operatives like “enemy” monks, wouldn’t your riot police have trouble which heads to bash and which to leave alone?
Note too: none of the footage shown on Chinese Central Television showed monks or nuns committing acts of violence. If Beijing is really so anti-religious and atheism-loving, wouldn’t they be giving maximum air time to such footage?