I have to agree with LearAv8r here,
There is actually a lot to towing a sign, keeping it readable from the ground, the aircraft I was flying where all highly modified. J-3 fuselage with super cub tail, wing extetions, leading edge cuffs, two sets of stall fences VG's on wings and tails, and 180 to 235 HP lycs. Also towing signs up to and including 70' X 110'
If you put someone in one of those airplanes, and hook them up to a sign that size without proper training, One of three things will happen.. They will rip the sign on pick-up, when they feel the jerk of a sign that large they will kick it loose, or they will get to slow at the top of the pick up, and enjoy a little stall spin at 200'. I've seen it happen and drug the body out of the airplane.
So small signs, with mostly stock airplanes is one thing that may not require a whole lot of training. But it also doesn't make a whole lot of money. So, if you are doing to tow for a stable company, with good contracts, you are going to be flying big signs in modified aircraft. So you need the training. And having the endorsement makes it easy for the local FSDO to add you to a waiver.