The term "weekend warrior" is generally used with a negative connotation, usually refer to someone who does something outside their normal sphere occasionally enough to not really know what they're doing. It has also taken on a more positive context in recent times, with people embracing the term and using it as a point of pride.
They refer to themselves as weekend warriors because it makes them feel adventurous - those for whom the particular activity is a way of life use the term negatively in regards to the same group. Like most group names, it depends upon who's saying it.
The term originated in reference to the military reserves, with their commitment to "one weekend a year, two weeks a month" of service. They were literally weekend warriors - holding regular day jobs, then becoming a soldier for a brief period to fulfill their commitment.
In this light, the term was a positive - it indicated that a "normal" everyday person could still serve their country, even if circumstances did not make service a viable career choice or long-term commitment. It implied bravery, patriotism and willingness to sacrifice, even if the terms of commitment were not full-time.
The negative connotation may have originated from full-time members of the military who felt that the "weekend warrior's" sacrifice and commitment to country was inferior to their own.
However the negative connotation began, it spread to many other walks of life to denote those "regular" people who spent time doing "irregular" things.
Non-athletes who suddenly decide to run marathons, businessmen who wear leather and ride motorcycles on the weekends, homeowners who take up DIY renovation, learning as they go, even straight-laced students who party excessively every Saturday - all of these people are included under the "weekend warrior" moniker.
This is because they choose to step outside their ordinary routines. It's unclear why this departure merits its own sub-population, rather than just referring to the new activity in context as simply part of the person's life. It may, perhaps, have something to do with the human inclination to belong to a group.
Weekend warriors who refer to themselves as such are proud to be a part of something new. Often, in order to participate in the new activity, a person must step outside his or her normal group.
If the new activity is a radical departure from the social norms of that group, some distance results - over time, as interest in the new activity grows, the person may begin to feel isolated from the original group and seek another group to fill the gap.
The person does not fit fully into the group associated with the new activity, however, due to the difference in immersion between lifestyle and hobby. So the person identifies with the "weekend warrior" group, which is both larger and smaller than either of the other groups.
Smaller because the subset of people pursuing that particular activity as a hobby is, necessarily, smaller than either those not pursuing it or those pursuing it wholeheartedly. Larger because, overall, the group pursuing any activity as a hobby, i.e., the weekend warrior group as a whole, is vast.
The people who pursue an activity "full-time", or as a lifestyle/career, tend to see the incursion of hobbyists in a negative light. Sometimes, the reason is justified - in the case of sedentary people suddenly training for extreme endurance sports, the risk of injury is high because their bodies are not acclimated to the demands of the activity.
In the case of the DIY renovators, by doing the work themselves, they withhold business from those who do it for a living - it's easy to see how resentment could arise, especially as the popularity of the DIY movement grows. With other groups though, the resentment occurs because of their own group identity.
The case of the businessman biker is a perfect illustration: the classic "biker" is a stereotype, but those who live that lifestyle live a certain way. It is not for everyone, so those who live that way all the time form a sort of kinship - they have sacrificed certain things to live a certain way.
When someone comes along who has not made those sacrifices and still gets to experience the perks of the lifestyle "part-time", there is a sense of resentment. Each party shares certain interests and desires membership in the group, yet they may remind each other of paths not taken. So the group splinters into the "full-time" and the "weekend warriors".
That's not to say that weekend warriors are either good or bad. It's simply a subset of the population - a sort of experiential tourism. There is definitely a divide between those that do and those that pretend, but as long as the groups can coexist and perhaps even learn from one another, division does not have to equal segregation.