First off, what does the composite output of the R15 look like? Is it correct when plugged directly into the TV?

Assuming everything there is fine, then the recorder will be recording it full-frame on the disc. If it's outputting such a recording vertically shrunk, then for some reason the recorder thinks the content on the disc is anamorphic. Black bars on the top and bottom is the correct behavior for an anamorphic disc set to play back on a 4:3 TV.

Technically I don't think there should be any difference between 4:3 letterbox and 4:3 pan & scan modes, since that reflects a DVD feature that was never implemented (the idea being that a pan & scan version of a movie could be automatically created in the player from a single widescreen copy of the film).

So, why does the recorder think it's anamorphic: possibly because the R15 is telling it so. Sony recorders (and other related devices) will recognize several embedded flags in a video signal, telling it that the content is anamorphic widescreen or even non-anamorphic widescreen (so with a Sony DVD plugged into a Sony TV, the TV will automatically know whether to use 4:3, Zoom or Full modes).

What you can do about it: if the R15 is at fault, make sure all of its output settings are correct. If they are, try testing the recorder with some other device that isn't 16x9 aware, such as plain TV output or maybe a VCR's output, to see if those recordings have the same issue. If they do, then the recorder is making all recordings with the anamorphic flag, which is wrong, but you may as well leave it set to 16x9 output to get a "correct" image. This may or may not affect playback on other DVD players depending on whether they do anything with the flags.

If recordings from other sources are fine, and if the DVD-R is designed correctly they should be, then the R15 is at fault and, assuming there's no settings on it that can be changed, then the solution will be the same... leave the recorder in 16x9 mode and complain to DirecTV.