True, but they can still be dangerous. Many can have serious side effects, but if they are sued then they "settle" with the complainant, and it never becomes public.
The Consumer Reports "Dirty Dozen" most dangerous supplements includes bitter orange, an ingredient found in common OTC diet pills.
Many diet pill ingredients can cause kidney problems, stroke, heart irregularities, conflict with prescription medications, and so on. None are without risk.
This Q/A on taking diet pills for energy might be of help