Mercedes customers given 2016 F1 power units

[CNR]

Mercedes customers given 2016 F1 power units

http://www.aol.co.uk/sport/2016/02/21/mercedes-customers-given-2016-f1-power-units/

"Mercedes have confirmed that their customers on the grid at Formula One's season-opening race in Australia will have the same power units as the reigning constructors' champions."

so why is this news was it true that "Massa: With the same engine as Mercedes we can close the gap" ?
http://www.grandprix247.com/2015/11/19/massa-with-the-same-engine-as-mercedes-we-can-close-the-gap/
"Felipe Massa is hoping Williams will get back on par with its engine supplier Mercedes to tackle the 2016 Formula 1 season on equal footing to the world champions.

While dominant Mercedes and its customer teams are believed to have enjoyed identical equipment in 2014, Williams, Lotus and Force India fell one specification behind the German carmaker in the middle of this season. ?

"
https://www.formula1.com/content/fom-website/en/latest/headlines/2015/12/path-cleared-for-Toro-Rosso-Ferrari-engine-deal.html
given that Current regulations do not permit manufacturer's to supply two different specifications of power unit over a single season, but Ferrari have now been given dispensation to supply a fourth customer team with year-old engines. how the hell did Mercedes get away with it ?
https://www.formula1.com/content/fom-website/en/latest/headlines/2015/12/path-cleared-for-Toro-Rosso-Ferrari-engine-deal.html

[AndyL]

Last year's regulations didn't permit the supply of more than one specification of power unit. The current regulations do. Apparently Mercedes have decided not to take advantage of that change. I don't see that they've got away with anything.

[Jag_Warrior]

While dominant Mercedes and its customer teams are believed to have enjoyed identical equipment in 2014, Williams, Lotus and Force India fell one specification behind the German carmaker in the middle of this season.

I think the same was true of Sauber and Ferrari last year. And no, I didn't understand that either. I also don't understand how Honda is able to avoid supplying other teams. Despite what their deal with McLaren might be, I thought there were FIA requirements that could be enforced. While I understand that if a driver uses too many engines, that driver gets a 500 place grid penalty, I don't completely understand what the rules are on the supply side.