Can ATC decide whether or not to provide tower enroute services?
According to the AIM, 4-1-19-c:


  There are no unique requirements upon pilots to use the TEC program. Normal flight plan filing procedures will ensure proper flight plan processing. Pilots should include the acronym “TEC” in the remarks section of the flight plan when requesting tower en route control. 


Based on that, it would seem that the TEC request was essentially equivalent to that of getting a pop-up IFR clearance. If you pre-file, the controller has all the information necessary to enter you into the system rather than asking for it over the radio. It wouldn't be legally required, but neither would the controller be required to issue a clearance if workload doesn't permit.

When using a TEC route, there is still the benefit of only talking to local controllers, and using a known routing that you're less likely to have amended.