When you're going at high speed (particularly when you're a kid on a jacky bike that doesn't handle well) taking that weird turn is a recipe for either crashing into the bush or losing your balance.
That, plus if there are any pedestrians using the path, they should get first dibs. Bicycles can ride on grass just fine, but anyone with a walker, a wheelchair, a stroller, or just wants to keep their shoes clean, should not feel like they have to squeeze or get off the pavement to let a bicyclist pass.
Here, the convenient option for bikes is also the most friendly option. Ornamental grass shouldn't even be a top-10 concern. If the municipality won't build proper infra for all users, then desire paths are all but guaranteed.