You can only publish about 4 times per day before you dilute the value of your upvote. So it is difficult for a whale to abuse this power.
Although whales will upvote their own Post, they generally don't upvote their own comments which they can do all day. If they did, they would be seen for doing it and confronted. If brought to the attention of somebody with a higher reputation than them, they can be warned against abusing the system in this way.
Whales are heavily invested in steem. The success of steemit is in their interests. It would be stupid of them to hurt the confidence of the community in steem by abusing the platform and judging by their success, I don't think they are stupid.
I guess that makes perfect sense ethically, but the excuse can't simply be that it's 'in their best interests' to act innocently within a fundamentally exploitable environment, audit-able or not, I've already noticed a couple bots being used for upvotes alone on content a couple whales care about because they don't want to over vote and dilute their value.