I think depending on your credit score, all the big companies may require a down payment, but then it's still 0%.
You're not locked into a contract, as you can pay off the entire balance at once if you want (but otherwise can't pay it off at a faster rate than the standard plan) but most people will effectively feel stuck to that service company because they don't want to pay the full balance. I'm sure that's why the company offers the installment plan without interest.
If you have no reason to switch service companies anyway, I'm not sure why you would feel the need to pay it all at once if it isn't costing you any extra by spacing it out.