As an example, I have around £18k worth of materials delivered to a job today.
I assume the risk for safe storage and liability for the payment due for the goods - title with supplier, risk with me.
When I pay the supplier the title passes to me - the goods are now my property.
My contract with the client specifies that title is retained (by me) until cleared payment is received in full.
The goods are on the clients premises so are at the clients risk (almost impossible to insure materials on a client's site).
Fairly standard business practise. Risk passes on delivery, title passes on payment.
I don't condone what the window supplier has done but I'd be reasonably sure that they have not committed either theft (the windows were their property) and criminal damage would be a matter of conjecture - removing some frame fixings wouldn't get the court too excited.
The youtube link earlier in the thread was (from memory) a customer who made no payment for a conservatory and porch. The builder approached the housing association (owners of the property) and it was agreed that he could remove the materials with the tenant being made responsible for making good.
As I say, I don't condone the action but do understand - it's not much fun being knocked!