Prior to 1976 two strokes were unabashedly allowed in the us, and it led to such legendary street bikes as the 750cc Kawasaki H2, 500cc Kawasaki H1, Rd350 and RD400, T500, GT750, GT550, etc...well you get the idea. Starting in 1976 the EPA began focusing on 2 stroke emissions for new vehicles and size limitations began to come into play. For a time the EPA choose to focus on the larger two strokes so utility equipment was unaffected. Also 2 strokes began to waine in popularity as 4 stroke street bikes began to get faster and more easy to ride.
By the 1980's in the US there was very little market for street going two stroke motorcycles. The companies that produced 2 stroke race replicas didn't import very many into the US (if at all). Smaller two strokes (250cc and under) stil flew under the radar, but the EPA was insistent that two strokes as a whole be cleaner, and pressured manufacturers of 2 stroke engines for cleaner designs.
Today 2 stroke engines are still legal in the US but they are not as common. For road going vehicles 150cc is the maximum and a catalytic converter is usually fitted to those bikes that still use them. until 2006 vespa still sold a 150cc two stroke, and Stella continues to sell them. The chinese scooters are a mix of two and four strokes, mostly 2 strokes for the smaller engines and four for the bigger.
With lawn equipment, the decision to build four stroke engines was a cost issue more than enviornmental, although the EPA does have restrictions on those smaller 2 strokes as well. With four stroke utility engines, the costs of repair, espically warranty repair are down. Traditionally, utility engines were two stroke so that they would be easier to fix in the field and not require as many specalized tools. But as technology improved the engines got more reliable to the point where a four stroke will often outlive its service life without any significant problem as long as it is well cared for. It is common to see a four stroke weedwacker and lawnmower these days, but if you really want two strokes can still be had.
I own a two year old two stroke snow blower. I have a closet two stroke fetish and where reliable I prefer my vehicles to have blue smoke trails behind them, although my 2 smoke snowblower rarely does due to its design.
Anyway, there is plenty of material on the EPA and regulating vehcile emissions on the web and in the library - this was just a summary off the top of my head from the research I did a while back on the economics of the hybrid car (specifically tax incentives). It was very boring.