It is good that my ISP with whom I have an email account filters out a lot of spam email, but why does it get to my account in the first place?

If I have a unique email address like myname@btinternet.com why do I get emails that are not addressed to myname?

I get email that is addressed to myanything@btinternet.com for example.

I did hear an explanation a long time ago, and I can understand why emails associated with websites can be set up to receive any email addressed to that website regardless of the prefix name, but I cannot undestand why ISPs like BT allow email through to my account that is not correctly addressed. If it is a unique address why should I receive email that does not contain the unique address?

Please explain again why this happens? Can ISP do anything to prevent it and is it about time they did?