If a tip is 10% of the total cost, that total cost is mostly fuel, wear and tear, consumables and hopefully some profit. Proportionally (depending on location) not that much of the overall payment goes to staff wages. So 10% of the total cost as a tip to the staff could well be 100%+ of their wages for that trip. Tipping as a portion of the wages bill would make some nice logical sense but I can see that it's unlikely to happen in this lifetime.
On the advance tipping, I'm sure there are more sophisticated articles about the links and lines between tipping and bribery but let me share my opinion. You might be interested in the official definition of bribery:
Giving or offering a benefit with the intention of influencing a person, to obtain a benefit not legitimately due.
Tipping in advance or promising tips to individual staff for things that are not normally included or available, especially for goods or services that damage the environment or degrade the other guests' experience sits very close to bribery for me. When travelling to countries where corruption is endemic, it would be nice to see tourists from first world destinations modelling good behaviour - paying legitimate fees to the company rather than the individual, paying attention to instructions, following the rules. And avoiding "let me do what I want and I'll make it worth your while" attitude that I'm sure we've all seen in action on occasion.