Laziness has nothing to do with it. It's about previous knowledge and habit. We want to (and CAN) accomodate the two different methods so why not? Also, "everybody will have to change that function"??? What function? it's new in GMS2 so nobody has to change anything and just use it as they wish with the option that suits them best.
Actually,
@hdarren is correct here. It is a basic principle of UI/UX design that it is more usable when the thing behaves as the user expects it to.
In this case, if you show a developer the game_set_speed function, whether they are the seconds or microseconds type, they will figure it out very quickly
if it takes one argument (say, in seconds). With that required second argument using global names as flags, it is virtually impossible for anyone to figure out how to use the function without seeing some kind of documentation.
This is just one reason that it is actually more usable and "accommodat[ing]" for
either type of user.
To prevent breaking changes,
I recommend making the second argument of game_set_speed optional, where the default is in seconds. This is the best of both worlds.