Alternately it could be seen as evidence that PowerPC assembly syntax is a mess. For anyone who doesn't know, the way it works with typical PowerPC assemblers is that instructions take unadorned numbers for all arguments, and determine whether they refer to registers or immediates based on the instruction: "li 1, 2" sets R1 to the immediate 2 ("load immediate"), while "mr 1, 2" sets R1 to the value of R2 ("move register"). And then because people find bare numbers confusing, you have includes that do "#define r1 1" or equivalent for each register, so when writing assembly manually you can write "mr r1, r2". But because these are just dumb macros, nothing stops you from writing "li r1, r2" - the assembler will just macro expand r2 to 2 and treat it as an immediate!

Other architectures have the R prefix as an intrinsic part of the syntax, so if you write R2 in a slot where the instruction requires an immediate, you'll just get an error. If PowerPC did that, you'd still need to remember the right inline assembly constraint letter for GCC, but getting it wrong would 'just' result in an unpredictable compile error when the compiler decided to use r0, not silent misbehavior.