Unfortunately, Flipkart and Amazon as default issues B2C Invoices, as 99% of the sales through those are from individual consumers. So you are not eligible for ITC.
I would not call it unethical. You may think this as your local supermarket store like More, Big Bazar where you do not get B2B invoices across the counters.
Sensing the growing requirement of B2B invoices, Amazon has started B2B services, where you need to register yourselves, and then you can purchase items and you will be issued invoices with your GSTIN mentioned, to avail ITC