Well, notebook manufacturers don't like it when users play with fan speed or other parameters that might damage the system. They believe their solutions are perfect, often built on the edge of what's possible in terms of performance/power (thermal). So they don't allow users to make changes to their design. There is a lot of laptops which users believe don't have sufficient cooling, or on the other hand overestimated noise.
I think that acceptable temperatures on notebooks are those, which don't trigger thermal throttling. However from my experience if you put a high load on most notebooks, the temperatures shoot into the upper limits (Tj,max) and thermal throttling is engaged. Some professional series are designed with better cooling to sustain a high load, and a lot of value series are not designed to withstand high load over extended time periods. But this is different between vendors and models.