I guess a manufacturer would come to a "standard" speed for their fixed speed roasters by way of extensive R & D.
In terms of a variable speed roasters, any particular drum speed will also depend on whether they also have a variable speed fan and variable burner (or not), because there will be any number of combinations of drum speed VS air speed VS drum temp., and that is where the challenge lies for the operator. A manufacturer may not (?) necessarily be able to recommend any particular drum speed, because it may be that there will be different burners specified for different markets as well as any number of other variables, which means there will be different behaviours displayed in any individual set up in toto (which is certainly the case with latest technology professional equipment where there are no "default" settings to start with).
It may well be, that's its not terribly helpful for manufacturers of small end roasting machines to give newcomers to coffee roasting too many variables to get themselves confused with, and I would rather see such roasting machines produced with no variable burners, drums or air speeds a la "old style" drum batch roasters....but it depends on how you want to look at it
In terms of small end roasting machines produced by manufacturers who themselves seem to be newcomers to this industry, it almost seems like a cop out to produce equipment where they can say to their clients......"we made it for you....now you work it out to suit what works best for you....."? Seems like a way of getting out of having to sort out the equipment and selling it as fit for purpose straight up to the type of market that the equipment is most probably pitched at (probably beginner level).
But of course that is just one opinion.