I have pretty much the same thoughts as yourself. For me a tip is an amount of money i feel the quality of service deserves, relative to what i can afford. Not a pre-determined amount. However, i can understand different cultures can have a different approach. That doesn't change what i can realistically afford though.
I have read the contract and they are asking for the amount above, it doesn't say i have to give it though. Apparently an envelope is passed around at the end of the trip. Whether this is done in private or not is another thing. I have been on many dive trips before, mostly in Asia where the dive guides earn very little money. I have never been asked for a tip, but i always give one if the efforts of the dive crew and staff warrants it. Never as much as the suggested tip in this contract though. The way they have constructed the sentence, sounds like they expect it although it's not a legal requirement (part of the cost of the trip).
A 15% tip is in the list of things to bring, seriously!
The thing is a $450 tip divided by say 8 crew members is about $60 each. So over a week liveaboard thats under $10 a day which is nothing to the individual receiving the tip, but a lot of money for me to pay out. I suppose it depends on how many crew mwmbers there actually are and how many guests. that $10 a day could be $100 a day if everyone gives what they are asking. I'm sure they are getting a wage as well.