Seizing and selling or destroying property used in the commission of a crime is common. It doesn't seem unreasonable to me if the value of the property is in proportion to the severity of the crime, and the owner of the property has been convicted of the crime (i.e. it's not civil asset forfeiture).
On the other hand: if you steal a pair of shoes, and the police find them back in your home, they don't get to burn down your house after your conviction.
Given how much of our lives happens on smartphones these days, willfully destroying personal data which is not in any way related to the crime seems like extrajudicial and unusual punishment to me, and not something the police should be able to do without an explicit court order.
It is common, and in this case the police were, according to the article, specifically not given permission to do so or did not seek permission to do so.