Most people have told me to run speaker level from the receiver front L/R into the binding posts on the sub. Why doesn't anyone use the sub line out from the receiver to the line in on the sub? What is the difference? Thanks.

most DO use the line out (LFE) from the sub if the AVR has it...

Some differences:

1. level of the signal...speaker level is high level ... line out is low level. whenever possible use line out if the signal is boosted again...no point in adding anymore distortion and circuitry than necessary

2. speaker level doesn't use the AVR built in bass management and relies on the subs crossovers. line out is crossed over at the avr.

3. line out is less prone to interference from other sources since the level is low.

Most people have told me to run speaker level from the receiver front L/R into the binding posts on the sub. Why doesn't anyone use the sub line out from the receiver to the line in on the sub? What is the difference? Thanks.
"Most people" may have told you that, but most sub manufacturers tell you to use the line out hook up. The high level (speaker) input is only there for those who do not have the option of using a line level (LFE or pre-out) connection. PLincoln listed the reasons why.