Core sets are known for making color pie violations - let's charitably call them bends rather than breaks - in the name of flavor. R&D overlord Mark Rosewater is known to oppose this practice (rather in vain, since he's never on core set design teams due to having to work on fall large expansions); the first sets new players are likely to pick up, he argues, should telegraph the color pie to them by having the strictest and most orthodox interpretation of that. Green flying creatures don't fall into color pie orthodoxy by any means - and yet, they appear in Magic 2015. But Rosewater acknowleged on his Tumblr blog that they may be less an outright violation and more a true shift in the color pie's layout.
When silenceuk asked, "So how upset do the [Insect] tokens made by [multiple Magic 2015 green cards] make you ? Flying death touch things in green ? Death touch is kinda in green isn't it ? But flying same big no no I thought," he said:
I feel like the hornets are slowly moving closer to my comfort zone. There’s still some room to go though.
The pro arguments seem to outweigh the con ones: Small deathtouchers are likely to be used defensively, and green can defend itself against flyers. The flavor of protecting the hive fits very well with green's ideas of nature, instinct, and community. And the fact that flying is one of the most often used and integral mechanics of the game means that green getting to have none gives it an unnecessary weakness atop the ones it already has (chiefly having to accomplish everything with creatures and combat rather than direct spell effects). The only things pushing back against it are green's historical antagonism to flying and concerns about giving one of the best evasion keywords in the game to the color that arguably should have the best (or at least most undercosted) creatures overall.