I think it would be more correct to say that garlic turns bitter when it is overcooked or burned; a touch of browning is not going to do that. Since it is usually sliced, minced, or crushed, overcooking it is easy to do in high heat methods.
Garlic, like many of the aliums family (including onions, shallots, scallions and so on) can present different flavors depending on how it is cooked.
The different cooking methods in the many cuisines of the world bring out different aspects of the flavor or use it differently. Most cooking methods reduce the sharpness or pungency of raw garlic. How long you cook depends on how much of the sharpness you desire.
- Raw, it has a sharp, pungent flavor
- When sauteed or fried quickly at the beginning of a dish (common in both Asian and Western methods), it releases its flavor into the cooking oil, which will then permeat the entire dish with its perfume, as well as cooking the pieces through so that they are softer and more palatable. The edges might brown a little bit, but they will not become a deep sienna color, indicating the burned flavor is present.
- Roasted, the garlic mellows and can be used in much larger quantities for its aromatic qualities, without the pungency.
Recipes where it is fried until crisp will direct you to only go to a light brown (for example, as in this recipe for gnoccii with crispy garlic—and of course, the garlic must be sliced quite thinly to do this. The goal is to get it crisp before it is overcooked, and avoid the burned flavors.
Garlic is normally added to recipes at the beginning of their preparation, in order to allow its flavor to permeat the entire dish. however, in a salad dressing or similar (especially in a raw applicaiton), it may be added right before service so that it is at its sharpest without the ... stale... taste that some aliums can get when chopped but not cooked for a longer time.