Correct. It's EPA mandated. Actually.....the regeneration system is NOT mandated, but the type and amount of exhaust emission is. How the manufacturer achieves that is up to them, and the cheapest least invasive way to do it is currently the system that almost all manufacturers use; and it don't matter if it's orange, green, yellow, pink, purple, or pepto pink with green and purple polka dots-every USA tractor is required by law to meet the same standards. At one point, there were credits available, which manufactuers could buy-which would prolong the process of converting to Tier IV standards. I think Mahindra is still doing some of the ~30hp stuff without a DPF but that wont' last forever either. Plus Mahindra is a pile of steaming feces as far as tractors go. And that's coming from someone who used to love them.
Under 25hp is not required to meet the same stringent emissions as the >25hp. So some companies have de-rated some tractors to, for instance, 24.8hp, to avoid putting it on their tractors-and that marketing tactic is apparently working. The Kubota L2501 is VERY popular-and no DPF and of course no regen cycle.
The 3301/3901 system is really simple in it's basic design. Think of it as a filter. The government don't want any soot to come out of diesel tailpipes (not since 2007.5 anyway), so basically all you have is a catalytic converter and a big screen type deal inside the muffler. The cat doesn't have anything to do with anything other than cleaning up some of the gases, just like cars have had since the 1970's. The DPF-or diesel particulate filter (diesel particulates are what we call soot) is just a high tech screen that catches the soot. Over time the soot builds up and it's got to be burned off. That is the "regeneration" (or regen) cycle. And it's simple.
Run the engine at or near full rpm and load, it won't regen very often because there's enough heat in the system to burn off the soot during regular operation. But not everyone runs their tractor that way. Many of us idle around. Start it cold, drive to the mailbox, shut it off, get the mail, fire it back and drive another 100 yard back to the house. It never gets hot. Soot builds up. Then you get a little light in the dash that is a picture of a engine with what looks like something going through it-that is the regen cycle lamp. When that comes on, just raise the RPM to nearly full throttle and it'll perform it's own regen while you are working the tractor. The only time you need a parked regen is if it starts beeping. Once an operator figures this out, it becomes seamless-and most of the time you don't even realize that it's doing it as it becomes second nature.