While other bases can undergo deamination (or other chemical changes) they do not give rise to another standard base and can immediately be seen as a mistake. For example adenine gets converted to hypoxanthine.

So, in a C-->U change in RNA there is no way to tell that anything happened, since both are naturally occurring bases in RNA. Any other change (A-->HX) is spotted as an abnormal base.

By contrast, in DNA, by using thymine instead of uracil, the C-->U is seen as a mistake as uracil should never be there 

I don’t know if this clarifies my point, or makes it even murkier