So to answer the question as to why a safety, this is THE reason why it's on the M&Ps. It really has nothing to do with lawyers or marketing or anything like that, the reason for the safety was the proposed 2006 US Army Pistol Trials, this is taken from the wiki entry of what the government wanted....

Overview of 2005 JCP Solicitation

This was based on original solicitation, which was later modified, and finally, heavily changed in March 2006, with the program renamed to Combat Pistol.
The notice starts: The USSOCOM intends to issue a solicitation to obtain commercially available non-developmental item (NDI) Joint Combat Pistol (JCP) system, Caliber .45 (ACP).
Two configurations required: One with no external safety and the other configuration will have an external safety.
The 'Combat Pistol System' is to consist of:
a Caliber .45 pistol (designed for A475 and AA18 rounds)
Magazines (standard and high-capacity);
Suppressor Attachment Kit
Holster
Magazine Holder (standard and high-capacity)
Cleaning Kit and Operator's Manual.
Estimates for max procurement quantities for the system are listed as
45,000 no external safety
600,000 JCP with the external safety configuration
649,000 Holsters
96,050 Standard Capacity Magazines
192,099 High Capacity Magazines
667,000 Magazine Holders
132,037 Suppressor attachment kits
While this is the max procurement, in comparison the initial order for M9 pistols was for 300,000 pistols (followed by more later).

I bold-ed the part that calls for the safety. The M&P was one of the pistols submitted but ultimately the whole thing was nixed do to cost cuts. The M&Ps sold in NY and CA (and a couple others) do NOT have the thumb safety because of a state requirement that I'm sorry I do not remember.

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This Signature is too Short!