Woohoo! 
Because for non-Golay barcodes, the orientation doesn't matter, so long as the mapping barcodes are in the same orientation as the barcode sequences. The computer literally does a line-by-line check for equality: is AAAC equal to AAAG? No, then this sequence isn't from Sample 1. With Golay error correction, the orientation matters 100% --- the barcode sequences and the mapping sequence both need to be in the 5′ -> 3′ orientation in order for 1) the error correction to work at all, and 2) to match the mapping barcode to the corrected barcode sequence.
Hope that helps! :qiime2: 