Inbreeding in humans is brother and sister producing kids. Same mother and sister.
As we are all human, does that mean same race eg. caucasian, asian etc. is inbreeding? I think not as the gene pool is vast for the same race.
However perhaps considering the auto strains are not that many generations old, they could be considered distant relatives at best.
Breeding can produce desirable and undesirable siblings. I am sure the breeders can comment on how they only choose the best parents(determined by traits they prefer) to improve the gene pool of a strain.
Only choosing from a small crop would produce less to choose from for breeding.
If you planted 100 or more plants, there would always be a small few that stand out in the crowd for breeding. If you only grow a few, less to choose from and possibly breed poorer siblings(over several generations, this would become more noticeable).
My point is, inbreeding or not, is not the real issue. Choice of breeding stock for a strain is more important.
Variability can be important, as it does not allow a poor trait to become too dominant in future breeding.
I said my bit, surely someone will cut me to shreds about my theory on breeding.
My 2 cents.