This attitude always arises when a developer decides to use the same game engine for the next installment.
Same thing happened to F:NV, people were crying that it wasn't an expansion pack to F3.
If it's a feature length game with enough new content to justify that label, it deserves to be full priced.
(But if it's pretty short and there aren't enough tweaks to keep the combat fresh, it probably could've been like a $30 DA:A type expansion)
The difference is that FNV was a different game with different mechanics and a different setting by different developers and basically a straight upgrade to F3
While BL presequel is BL2 in space