Giving IB/AP/Honors classes extra weight in high school is necessary to offset the additionally difficulty of these classes. Otherwise, high school students would have a direct disincentive to take advanced classes.

A swift googling brings up this forthcoming study of about 900 high schools in Texas:

Despite conventional wisdom to the contrary, grade weighting is not the primary factor driving students to increase their AP course-taking. Moreover, a lack of institutional knowledge about the importance of grade-weighting does not have a practically significant adverse impact on students with low historical participation rates in AP, although low income students are marginally less responsive to increases in the AP grade weight than others. The minimal connection between AP grade weights and course-taking behavior may explain why schools tinker with their weights, making changes in the hopes of finding the sweet spot that elicits the desired student AP-taking rates. The results presented here suggest that there is no sweet spot and that schools should look elsewhere for ways to increase participation in rigorous courses.

But there's still the additional incentive of prestige and signalling, isn't there? That should be enough for the serious scholar. It's a significant problem when non-AP-labelled courses are often passed over for the purpose of a cheap grade boost.