Akshan's indicator is there for a couple reasons (that are mostly consistent with the other champs you mentioned):
- This is an ultimate level execute ability, so it having more consistency (through an indicator) makes it more exciting rather than frustrating for the user.
- Akshan R has variable charge time and you can easily be punished for charging too long (the opponent can access the counterplay by ducking behind cover). Having the indicator gives the Akshan a clear indication of how long to charge without having to have X games on the character to feel out all the breakpoints.
- Akshan R indicator does not turn on before you start casting. Akshan is already commiting to a cast before he even gets it.
- Other champs (like Kalista) have very low penalty to "missing" the execute threshold, in both cooldown and damage lost.