I guess for McDo the sauce costs a penny or two and they want to minimise interactions as they are costly; but theft must be too high off sauce is freely available.
For McDo it might well be a strong differentiator, customers see them as generous because of the excess sauce. Might also cause customers to make less mess under the psychology of feeling endebted when receiving a "gift".
Re plastic, a packaging company CEO stated on BBC radio (R4) this evening the expected additional cost of replacing burger packaging with fully home-compostable burger packaging was around 1p and that they had similar solutions for all fast food packaging ready to go. Financial incentives to responsible packaging solutions seem necessary at this point.