Everyone knows that the traditional German style trombone carries the word "garland" (kranz), but why is this type of garland removed by most brands in American style instruments? What kind of effect will it add to the trombone tone?

Not all German style trombones have a garland. Depends on how thick the bell walls on the particular design. The thickest bells don't have a garland, some get thin enough at the edge to benefit from a narrow garland, and the thinnest have very wide garlands.

Its purpose when it's there is to reinforce the bell, because it is very thin. Usually paper-thin beneath the garland, thin enough that you can bend or crimple it with your fingers with little effort. Bells with thicker walls don't have the garland, because they don't need the reinforcement, and the extra mass is eventually detrimental to how it plays. So an American-style bell with a Garland would make no sense, unless it was made much thinner than usual.

[quote="LeTromboniste"]Not all German style trombones have a garland. Depends on how thick the bell walls on the particular design. The thickest bells don't have a garland, some get thin enough at the edge to benefit from a narrow garland, and the thinnest have very wide garlands.[/quote]
It may also depend on the hardness of the bell material. My Cerveny 781 BBb tuba was red brass (a 90/10 alloy, and a real dent magnet). It had a nickel bell Kranz/garland. Most Cerveny tubas with yellow brass bells don't have a garland, but a couple do. On the other hand, Cerveny has a few tubas with red brass bells and yellow brass bodies where the bells don't have a garland.

Some makers of ("French") horns (e.g., Schmid) employ a garland to affect how the horn transitions to a more "brassy" sound

The B&S trombones have versions with and without the silver garland. The ones with the garland play a little more centered. I don't think they changed the bell weight between the two versions, but you could email them and ask.