This is mostly a manufacturers restriction.
The profit margins in the music game are large, there is never a shortage of folk wanting a slice of the action so importers can set the conditions of sale they want, things like not allowing any returns back to the manufacturer at all, so the end seller having to carry all the burden of warranty returns is not unusual.
The guitar industry is not particularly fragmented either, it is dominated by two or three huge major players that have several differant brands, so that Jackson, Charvell, and the cheapo Hondo are all part of the same group Samick, despite catering to vastly differing markets. In the case of Samick they also own a lot of other brands such as Kahler, Gorilla, Grover etc along with a wide range of other electronic devices, so if you upset this conglomerate, not only do you lose the right to sell their guitars, you also lose the right to sell a huge range of other stuff too, which could easily mean the end of your business.
Floyd Rose and Kahler, despite appearing to be competing for the same market, are not as far apart from each other as you might think.
Guitars are very much cheaper in the US than the UK, and the manufacturers have artificially hyped up prices here, by at least one third higher for the same spec instrument, and this sets up a situation that is ripe for parallel imports and grey imports.
Any dealer found out soon finds they can’t get stock from anywhere at all, the big manufacturers somehow seem to not be able to sell to anyone who has upset one of the other big manufacturers.
What I’m really saying is that in many ways, the musical instrument industry operates like a cartel, they do not compete as aggressively against each other as you might suppose, smaller manufacturers that do well don’t tend to stay independant for long, they get bought out.
Those smaller custom makers may be very well known, but in terms of the value of goods they shift, its a vanishingly small percentage compared to the big manufacturers. Some seemingly competing brands are made in the same factories in places such as Korea, but they are made for differant music merchandising corperations, examples such as,
The largest guitar maker Samick have been badging their guitars as “Epiphone/Gibson”, “Washburn”, “Hamer”, “Slammer”, “Kima”, “Danelectro”, “Archer”, “Ibanez”, “Rogue”, “Hohner”, “Slammer”, “Memphis”, “Montana”, “Grand”, “Dean”, “Johnson”, “Commodore”, “Hondo”, “Cruise”, “Kramer”, “Karera”, “Sterling”, “Lyon”, “Mitchell”, “Music Drive”, “Santa Rosa”, “Jay Turser”, “DeArmond”, “Abilene”, “Shredder”, “Sigma”, “Sebring”, “Eagle”, “Rokker”, “Joshua”, “Mr. Potato”
That list would give many guitarists a bit of a shock, all those names made in just one factory and just one manufacturer shows just what a stranglehold that these companies have and its why they can dictate terms.