Why are they better?  Why are they stronger? And why do you think they're "free"?
Because none of those facts are true. 
unless there's been a huge jump in the technology, compostable bags, especially those made from corn starch, cannot match the strength and durability of good ol plastic bags. . .since they are stronger, they can hold more and last longer, and are, thus, better.  (i had a similar discussion with someone about how crappy compostable utensils are compared to plastic utensils.)
as for free. . .let's see. . .am i willing to pay 1/4 of a cent more in higher prices for the "free" bag versus paying $3 for 10 compostable bags?  i say yes.