Crack the crab legs to expose some but not all of the meat. Collect any liquid that might come out of the shells.
Heat the vegetable oil over medium-high heat in a large skillet until it shimmers. Add the garlic cloves and crushed red pepper, and saute until the garlic just begins to brown. Add the crabs and cook until the shells turn red, tossing constantly to let the flavored oil hit all parts of the crab.
Remove the crab pieces and set aside. Pour off the excess oil, lower the heat to medium, and add the sesame oil and ginger. Cook until the pan contents are very fragrant, then add the pineapple juice, soy sauce, fish sauce, and the liquid that you might have found inside the shells in step 1. Bring to a boil and stir until the liquid is reduced by about a third.
Return the crab pieces to the pan and add the tomatoes. Cook for about five minutes, until the crab is heated all the way through. Add the lime juice and cilantro, and serve right away with cold beer.