When setting up a blanket agreement you do not yet know the exact location that goods are being received into. Yes there appears to be validation that means a blanket agreement must have the correct location set at the header level (it ignores the line which appears to be a defect).

No one should know where goods are being used at the time a blanket agreement is set up for purchasing. These fields should be removed from the page and from validation, with the existing location on the actual sales and purchase lines being the only location which matters.