Why do some people say that people with true ADHD don't get addicted to medications that are stimulants?

Not really correct. Anyone can abuse any med that is abusable (e.g., opioid/morphine derivatives, methadone, amphetamines, methylphenidate, benzo's, barbiturates, muscle relaxants such as carisoprodol, etc.). About 7 million americans report abusing prescription drugs each month (nida, 2010). One theory is that if you are truly adhd & you take the adhd med correctly & you function better, abuse is less likely.
Monitoring. Studies have shown that people with ADHD, who take stimulants as part of a comprehensive educational plan, with a doctor monitorning the medication use, have a much lower rate of addiction of abuse of drugs and alcohol than those with ADHD who are self medicators or those who take stimulants illegetimately, especially if they are mixed with other drugs of abuse.