Negation is different because it's entirely ambiguous. You can dish them out freely and retroactively rationalize your intention. -1's are way more susceptible to knee-jerk emotional reactions than +1's.
A relevant example. Think of the political candidate you like the most. If someone you really like asks you to +1 a persuasive argument about their competitor, would you? On the other hand, if they asked you to -1 someone, you probably would. Even if you knew very little about them.