Well, it's more that for those bought devices or financed devices there's a discount that the carrier offers on the current plans, while there isn't one for contract phones, or for older plans.
What that means is that on older plans that don't offer that discount it's cheaper to buy a contract phone since they would be paying the same thing on a monthly basis whether they bought a discounted phone or paid full price for one. Even for some on current plans, depending on the discount they would get (since it can be as little as $15/month or as much as $25/month) it might still be less expensive to get a contract phone.
All of this applies mostly to AT&T and Verizon as those have fairly similarly structured plans and pricing.
True, if you're on a contract plan. My point is simply that going off-contract from the outset allows for the flexibility to switch into a better plan when the offers become available, rather than waiting until the end of a two-year contract.
I bought my iPhone (paid in full) in September 2013, and the plan options that became available not long afterwards are way better than what I had to choose from when I bought my phone. If I had locked myself into a two-year AT&T contract at that time, I'd still be locked in and would end up paying over $2,500 by the end of the contract. Going with AT&T's current plan options would save more than $200 over two years. Going with the prepaid T-Mobile plan that I ended up choosing will save me more than $1,000 over two years.
Yes, if you go with a contract right now, the cost would end up about even compared to an individual Mobile Share plan with AT&T Next. But, a contract would eliminate the option of switching to any better options that come along over the next two years. Given the rapid pace of change in the telecom industry, I would guess that better options are very likely before today's two-year contracts expire.