Both authors and voters don't have to buy STEEM if they signed up via Steemit. There are many free accounts using Steemvoter service (while they barely get curation reward, they are doing because they can earn anyway).

Generally, writing a post consumes tens of times of time and energy more than reading, or infinite times more than bots because they don't read. While, there is an argument for tipping-based system as well and I understand it's reasons, I would say rewarding author is essential.

Bad voting doesn't directly decrease the price nor good voting increase it. But degraded platform by bad voting can drive away users and consequently harm the price.