This is a fallacy born of being myopic and motivated by unreasonable optimism. Not only does it neglect a credible rather detailed witness report of a red pick up truck (the timing of which fits her accident exactly) but it also glosses over Maura Murray's motivations at this specific point in time.
Maura was highly likely drinking red wine from a small plastic coca-cola bottle at the time she had her accident. This car accident is her second (or possibly third if you link her to the Varsi situation) within just a couple of days. Maura asked Butch Atwood not to call the police when he responded but she probably realized they were already on the way due to the inhabited properties opposite her. It seems clear and logical to me that the most important thing for Maura at this point in time is to avoid the Police at all costs. Where she is there are really only two ways that Maura can avoid LE. Maura can run into the woods, thereby leaving tracks in the snow which in turn leaves a chance of her being located. Alternatively Maura can flag down a passing motorist, this has to be the most effective way of leaving the scene at speed.
Consequently I believe that instead of being snatched by Fred's titular 'dirtbag' Maura might well have flagged him down in her desperation to leave the immediate area. If Maura happened on another good Samaritan like Atwood then we would have a lead, as we do not I can only assume the driver had bad intentions. If we believe the witness about the red pick up then it would be passing Maura just as she would be wanting a ride out of the area. Maura might never usually get into a strangers car but it looks like she has already been drinking behind the wheel and might be partially intoxicated. If Maura feels depressed or suicidal this also heightens risk taking activity.
Now if Maura did hitch out of that area in the first vehicle that picked her up (and I did read that Atwood saw more than a few cars go past while waiting for the police) then she initially went willingly and unfortunately might be anywhere. Perhaps a reason people are keen to discount this theory?
I wonder sometimes if we tend to get lost down the rabbit hole in trying to work out exactly what Maura was thinking during odd incidents in the days before her vanishing. Understanding, say, Maura's motivations after she gets the call that terribly upsets her at work would be hard enough if we knew exactly what was said in the call. Not knowing what was said in the call makes the challenge practically impossible. Add to this that people are often illogical and act in ways that even they do not understand. At any rate, keep up the search and surely discussion can only be a good thing as it keeps attention on Maura who, I am sure, remains in all our thoughts.
Strident verbiage aside, I think your point concerning Maura's desire to avoid contact with law enforcement is spot on.
However, my assertion that Maura was a victim of kidnapping/homicide is borne not out of optimism or 'myopic vision' but fact. According to city-data.com, there were a total of 2 murders in Haverhill from 2004-2012, with zero murders being reported in 2004. That statistic alone makes it unlikely that Maura was a victim of murder.
It's much more likely that an inebriated, terrified, possibly hysterical Maura stumbled into the woods, maybe only intending to wait it out until LE came and left and that she was then incapacitated by the elements or by sustaining an injury. Those woods could be searched 1,000 times without someone finding her remains. I'm so sorry for Maura and for her family that whatever transpired in those days before her leaving campus ended in terrible tragedy.