For safety reasons

The United States is a very litigious society. If a child is injured, or heaven forbid, killed by use or abuse of a toy, that child's parents are very likely to sue the toy manufacturer. As such, Hasbro puts its toys through rigorous safety testing. Sometimes, a toy has to be altered from the original design in order to maintain safety standards, which vary from country to country.

Safety testing
Hasbro has several tests that come up frequently. One of the more famous ones is the "drop test." As its name implies, this involves dropping a toy from great heights in order to ascertain whether or not said toy will break, and if any of the parts which may have broken loose could potentially injure a child (sharp edges, swallowable/chokeable bits, etc.). This is often cited as the reason Fortress Maximus has not been reissued in the United States, as well as the shortened smokestacks on the shoulders of various Optimus Prime toys (which statred back in 1988 with Powermaster Optimus, in actuality).

Other tests have similar goals, generally concerned with breakage issues. Hence, the Japanese version of Vector Prime has hard plastic wings, but the U.S. version uses a rubbery material less likely to accidentally gouge out an eye. Even as early as 1985, Jetfire was altered to have a blunted nosecone, and the Seeker jets had rubber nosecones instead of hard plastic. Again, this was likely to prevent trauma from a thrown or jabbed toy.

Safety standards
Besides the safety testing, Hasbro (and other toy manufacturers) try to make sure their products conform to certain standards. Some of these standards are self-imposed, while others are mandated at the state or national level. Most notably, these laws include choke laws, which are designed to prevent small parts (especially projectiles) from being lodged in a child's windpipe; the use of toxic materials like lead-based paints; and toy gun laws, which are designed to prevent scenarios where law enforcement officials accidentally shoot children or adults who are not armed, but carrying "realistic" toy guns (like say, G1 Megatron. U.S. law requires that toy guns have either an orange plug in the barrel, or a coat of orange paint on the barrel.  Some states have even more stringent laws, which requires that toy guns must be brightly colored and must not resemble real-world firearms.  (Some retailers won't carry realistic toy guns anyway, so that's a double-whammy some places.)

Trivia

 * According to Hasbro, toys that represent flying characters are given more stringent drop tests and rounded/collapsable bits, as children are more prone to throwing these figures around or ramming them into things (or people).


 * In Italy's Transformers line, the missile launchers didn't have their springs removed, but the missiles did have comically-huge giant rubber balls attached to the tips. Makes those Comemmorative Series missiles not seem so bad, huh?