No idea why its there, but here's how to disable it with the GNU implementation of bc:
echo '6^6^3' | BC_LINE_LENGTH=0 bc
BC_LINE_LENGTH
This should be an integer specifying the number of characters in an output line for numbers. This includes the backslash and newline characters for long numbers. As an extension, the value of zero disables the multi-line feature. Any other value of this variable that is less than 3 sets the line length to 70.
