Well I think sometimes the media doesn't make it clear what is actually going on in the minutes and hours after such an incident.
The police presumably have to assume worst case scenario. Therefore, if something like that happens, they:
- Assume that it is a terrorist incident.
- Assume that there are multiple perpetrators and that some may still be at large.
- Assume that there may be other attacks planned to coordinate with this one.
- Assume that different scenes nearby were committed by different attackers and no the same one.
This is obviously the only logical thing to do, otherwise if the above turned out to be true, they would be accused of incompetence later on.
Therefore they say that they are "treating this attack as a terrorist incident".
Later on, it may actually turn out that it wasn't really a terrorist incident but a lone person who had gone crazy. By the time that becomes clear, a lot of people will have moved on and will always forever think it was a terrorist attack.
There are more people in the world who are Muslim than any other religion, so it stands to reason that some of them would be criminals or violent people regardless of whether they are Muslim or not.
Also, if somebody who is mentally unstable and is probably going to commit a violent act, then goes out and attacks people, and also happens to have associated with religious extremists at one time or another, is that really a terrorist act? Possibly they would have done it anyway for some other reason. Unless you can show that they are actually taking instructions or being organized somehow by a terrorist organization, is it really a terrorist act or actually just an act of senseless murder?