You are correct. I compare part 141 schools with FTOs in the UK. Part 61 is your 'bog-standard' registered facility. Because training under part 141 is done under more controlled circumstances, the dispensation exists. The same licence is issued in the end though.
Part 141 Eligibility for enrollment. A person must hold at least a private pilot certificate with an aircraft category and class rating appropriate to the instrument rating for which the course applies prior to enrolling in the flight portion of the instrument rating course.
Part 61
Aeronautical experience. A person who applies for an instrument rating must have logged the following:
(1) At least 50 hours of cross-country flight time as pilot in command, of which at least 10 hours must be in airplanes for an instrument -- airplane rating; and
(2) A total of 40 hours of actual or simulated instrument time on the areas of operation of this section, to include--
(i) At least 15 hours of instrument flight training from an authorized instructor in the aircraft category for which the instrument rating is sought...
Ref:
http://rgl.faa.gov/Regulatory_and_Gu...e?OpenFrameSet