They just don't make em like they used to.
Curious, do you have one of those sideways motors with the transmission in the oil pan ? I dunno but ..That might be a bad thing to tow with all the mechanical clunking going on that an automatic transmission would have more freewheeling gush.

This towing with automatic versus manual transmissions showed up as far back as 1978. As I recall, when Chrysler introduced the econobox Omni/Horizon vehicles with the little 4 clinder engines, these had the same recommendation: Towing recommended only with the automatic transmission. It had to do with the sustained rating of the transmissions and clutch combinations. I'm not sure which details are causing the restriction, but I can hazard a guess.

There isn't much room for the clutch/transmission assembly in front drive autos. Typcially, automatic trasnmissions use sun-and-planet gear sets while manual transmissions use the usual simple gear sets, either of spur or helical teeth arranged on a pair of shafts. A tpical planetary gear set has three planetary gears meshing with a central "sun" gear, thus the load is shared among three sets of gear teeth per stage, whereas for the simple gear set, there is only one set of teeth to carry the load. The planet-sun gear set has the potential to carry heavier loads than a simple gear set depending on tooth design and size.

The second factor may be a clutch limitation. The clutch must start both the car and tow load from a standstill. This involves a considerable amount ot slippage, then finally gripping tigthly and locking the cranksaft to the drive train. Clutches contain a set of compression springs arranged in a circle to act as torque pulse absorbers. These springs are preloaded and do not flex until torque exceeds the preload value. Under crusing conditions, these torque springs do not flex, but stay at their prelaoded length. Add an additional tow load and these springs may come into play while pulling a hill or accelerating. This extra flexing is beyond their intended operating condition and may cause clutch assembly failure.

The standard clutch supplied for a small car simply may not have the rating to start up a load larger than the vehicle itself, and when so used, its life is shortened. Also, there may not be enough room to enlarge the clutch for a larger rating. It may be a matter of ecomonics for the manufacturer. He may have decided that small cars can't tow very large loads anyway, and believes that, in his market, most owners don't buy these cars for towing, therefore, he decides to design only one of the transmissions for some towing capability.

In regard to gear ratio differences, I haven't investigated today's offerings, but I remember past practices. Before "lock-up" torque convertors, for a given car model, the final ratio for the automatic was a little "higher geared", e.g., assuming if there were no slip, the engine truned less revs per wheel rotation for automatics than for manuals. However, when crusing, there is about 5% to 10% speed difference between the input and output of the torque convertor. The final gear ratio was adjusted so that at steady cruise on a flat terrain, the engine rpms were comparable between an automatic or manual equipped vehicle. The adjustment was made in the differential gear set. The differentials for automatics were higher geared than for a manuals.

However, the use of a lock-up clutch on the torque convertor puts a new slant on gearing theory. The lockup clutch eliminates torque convertor slip at cruising.

It might be just me, but when I think "high" "tall" "steep" gear ratios I think more motor revs for less wheel movemnt. Old muscle cars typically came with 3.90's or 4.11 for example as opposed to "highway gears" like 2.70's which produce a higher top speed but less pull.

That being said manuals are typically rear geared "higher" than thier automatic counterparts (hence its common for auto to have 4 transmission gears and manuals to have 5-6 transmission gears). Following the logic the higher geared mauals would be better for towing not worse. I suspect as jem said its more related to the drivetrain's ability to handle the load than to the actual drive ratios. Maybe, manufacturers trust the automatic to shift more appropriately under load than the typical driver would manually, and the disclaimer limits thier liability under warranty coverage.

My 0.02.

Also, not for anything, but I certainly wouldn't tow much with a 4-banger cavalier anyway.