Because you can potentially find some non-infringing proxy for the infringing tests. If you demonstrate to the FDA that your proxy acts as an adequate manufacturing control, then you are done. Basically you have another degree of freedom in designing around the MNTA patent portfolio.
Now of course we don't know if Amphastar actually went down this path or not, but the fact that they are now citing the R&D exemption suggests that they might have.