Their phones were to be surrendered as part of an investigation. Wiping them is destroying records.

It’s different if you’re just returning a work phone, or preparing to sell or transfer a personal phone.

When members of a group do it more or less en masse, it implies that they have something to hide.

As a routine practice, yes. If my phone was subpoenaed (or likely to be in the future), wiping my phone would be against the law. I would only do so if the risk was worth it.