All tire compounds will crack at different stages in their life and different places on the sidewall due to several factors:
although tires for trucks/cars can crack at the point where the tread joins the sidewall, I've never seen it on a serviceable M/C tire.
excessive cleaning; there are anti-oxidants and anti-ozone protection that are leached out of the tire each time it's cleaned. the harsher the solution and solvents, and method, the more the leaching is accelerated.
heat, exhaust, ozone and sunlight all deteriorate the rubber compound. parking near a running air compressor, or generator = ozone due to the physics of armatures and electricity.
the anti-aging chemicals used in the rubber compounds are more effective when the tire is "exercised" on a frequent basis. The repeated stretching of the rubber compound within it's designed stretch limits actually helps resist cracks forming. low mileage and long storage is actually a detriment. nothing has ever been put in front of me that explained what the stretch limits are/were and how to measure. I do know this, some tire installers will over inflate to seat the tire on the rim and let it sit, supposedly to 'cure' the installation before letting the air out to spec. Not a good method for avoiding sidewall cracking.
in addition to under inflation, don't overlook over inflation. it will stretch the sidewall at the same spot next to the rim as the posted photos show.
the cracks can be cosmetic if they aren't too deep. how you decide what is too deep has never been explained to me. it's gut without special equipment.
auto tire manufacturer's generally have a four year crack warranty. I can't speak to M/C because I never checked.
