Because they have their own warchest of patents from acquiring about every decent processor startup over the past 15 years that would allow them to counter sue, and microarchitectual details are under incredibly strict NDAs to where it's an uphill battle to even prove that Apple is using any of those patented techniques to begin with.
Also, the PowerPC macs weren't their chips, those were IBM and Motorola for the most part (Apple did have some input into Altivec, but didn't do anything from the RTL down AFAIK).
You are contradicting yourself. Apple hasn’t been sued because of their patent portfolio, but they still have that. Little changes with this transition, they are still building computing devices around the ARM instruction set.
