I too always rip to straighten but I rip one end only and work from the rip end down. Then once I have my quilt sandwiched I cut off the extra. Yes extra wide backing because its folded then wound on bolt has a strong tendency to get uneven if they just lay it out on the table and cut. Its like cutting a plaid or stripe straight off the bolt versus cutting down the stripe. Burns me - I just lost 7" off a repeating plaid for this very reason.
For dramatic effect have you considered taking the ends in to explain to the store why you are upset? The visual of more than 1/2 yard might drive the point home quite quickly. I'm with the others on letting the store know or buying elsewhere.