The issue isn't nearly as simple as they are trying to make it out to be. Raising the rate of pay increases the cost of business for McDonald's; they can counter this by raising prices. Raising prices will reduce the amount of volume they sell (fewer people, even if it's just a small %, will buy the Big Mac or whatever example it was at $4.68 vs $3.99). Reducing the volume they sell will increase the unit cost of each item they buy for their producers. Now the producers are in the same boat; they can raise their prices and that will trickle down. Eventually the price increases will expand beyond the sphere of just McDonald's into other businesses that are buying the same commodities (corn, soda, wheat, whatever).
There's no way this stays isolated at McDonald's. Who would want to work at Burger King for $7.25 when the exact same job at McDonald's pays $15? The industry standard for a McDonald's type job would become $15/hour. Eventually, "better" jobs that are currently paying $15 an hour will have to increase their wages because who would rather do low end programming work or whatever at $15/hour when they can go flip burgers for the same wage? So those jobs move to say, $25/hour. Then the $25/hour jobs have to increase their wages etc etc. The price of everything will increase to compensate, wages across the board will increase to compensate. It's a never ending cycle that will just make it so that in a few years time $15/hour has the same real world value that $7.25 an hour or whatever has right now and in 2025 the same people will be protesting that $15/hour isn't a livable wage.
The real problem is that a minimum wage position at McDonald's requires no skills at all. It's not a job or role in society or whatever you want to call it that should be providing wages for people to raise families and live off of. It's a job designed to be part time, for young kids or students and the like. Management and what not makes more than a livable wage. The industry standard for restaurant manager level (at least in the Northeast) is around $45k/year. The instant you try to make it a career opportunity for the bottom of the chain, the whole system will rebalance around that. The simple fact is people in this position are always going to be making the absolute minimum and the true value of money is just going to decrease to compensate for any increase provided.