Does anyone understand "redeposit" bonus logic?

[otctrader]

I'm hardly an expert at poker site management, so maybe someone in the loop can chime in and explain why 90% of the bonuses we see are tied to redeposits instead of just instituting a playthrough requirement (and possibly creating incentives to maintain a balance)?

It's common sense, but redeposit requirements cause players to pull out funds for the sole purpose of redepositing for the next bonus, driving up transaction costs for the poker site and probably lowering their cash in circulation in aggregate when you factor in the periods between bonuses.

I'm specifically interested in this as it relates to the major sites, and not some fringe site running a ponzi scheme in dire need of cash infusion.

Discuss.