Some ambiguity in interpretation causes confusion. In addition to the passages you cited, I can think of one other in the Doctrine and Covenants with specific reference to meat:
And whoso forbiddeth to abstain from meats, that man should not eat the same, is not ordained of God; For, behold, the beasts of the field and the fowls of the air, and that which cometh of the earth, is ordained for the use of man for food and for raiment, and that he might have in abundance
Some argue that D&C 89 stipulates that meat should only be consumed in special circumstances such as emergencies or shortages, while others argue that D&C 49 specifically states that meat is ordained for man's use and should not be some extreme case.
The commas in verse 13 did not exist in the original text, and thus some have argued that the meaning of the passage is that it pleases the lord to use meat beyond the conditions outlined. Others argue that verse 12 specifically stipulates using meat sparingly, and thus the commas portray the true meaning.
While church leaders have given no specific official interpretation on meat in the Word of Wisdom, various leaders have offered their own interpretations, largely in favor of the latter interpretation above. Lorenzo Snow, the 5th successor to Joseph Smith, opined that members of the Church should become vegetarians. Heber J Grant offered his opinion that his longevity was in part due to his sparing use of meat. Many others emphasized that Latter-day Saints "eat little meat." I was unable to find any opinion supporting the former interpretation, however it is clear that meat is not prohibited based on the time of year or otherwise. Official church functions involving meals often include some meat as well.
Modern medical science has shown that, much like tobacco and alchohol, meat in excess causes a myriad of health problems, heart disease being perhaps the most common. This adds additional credence to the effectiveness of adhering to the Word of Wisdom, a health law predating modern science. Some meat, especially low fat and high protein meat, has shown to have positive health benefits. Perhaps it is for this reason that meat is not strictly prohibited, and indeed appears to be "ordained for the use of man."
Although there is much in Mormonism that is strictly defined, there is also much that is not; and it is left to each individual to determine the degree to which he or she must personally abstain from meat. Mormons believe that the Holy Spirit will guide individuals to know what God's will is for them specifically, not only in this matter, but also in any aspect of one's life.