It's not regulation for regulation's sake; it's to keep the next customer from an E15 customer from getting more ethanol percentage than they want/need. That part is simple but what to do about it is less so. Seems to me the <4 gallon purchaser has a choice: stick with a non-blender pump (I suppose at a busy station that could be a problem but otherwise not) or just use the blender pump and accept that you might end up with more than 10% ethanol in your purchase. I've not seen it but I don't think the pump or the station will make you pay for four gallons if you've pumped less than that.
You'd think it technologically possible to make a blender pump that doesn't contain residue from the prior purchase but evidently that's not where we're at.
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