Users of the internet should send traffic from their assigned address.
When they have multiple addresses they should use the address that belongs to the interface they send it on.
Either they route the traffict to the interface that belongs to an address, or they assign the source address depending
on the interface they want to route on.
Don't adhere to this rule and you face blacklisting of your traffic.
It is similar to open SMTP servers. Used to be no problem, used to be common practice, is not acceptible anymore today.