I don't think that is a fair statement. Often writers of emulators have to take low level shortcuts because the alternative is emulating the physical hardware in software - which is extremely slow. A "bug" implies that a behavior is unintended or undesired where as sometimes these kinds of emulator facets are deliberately implemented for the sake of simplifying the emulator design or just being about to use modern hardware to offload some of the processing (like a hypervisor would) rather than having to emulate every single transistor in a now defunct chipset.
You're right that emulators in general may do that, but MAME in particular is designed to be a faithful reference of the inner workings of the machines. Being able to play games is merely a "nice side effect".
There's a pretty clear difference of opinion regarding the purpose of emulators.
The creators of MAME are mainly interested in rigorously simulating certain kinds of hardware which happen to play games, just as a classic car restorer may rarely or never actually drive his meticulous reconstructions.
For many other emulator devs, and almost all emulator users, the point of emulators is to play games. That may not be your area of interest, but it's rather silly to say that they're just wrong.
There are limits to this, as with everything. No one is going to make their emulator 3x slower to properly emulate one glitchy line in Super Mario Bros. that shows up on 1/3 of powerons based on a random clock alignment issue.
Besides that, the real console hardware could vary more between revisions than the difference between a console and an accurate emulator, so you also have to ask whose real thing is being emulated? Even with Nintendo's attention to detail on lot check, some real Game Boys won't play Prehistorik Man. There are some NES accuracy tests out there that require making your emulator behave like one specific console owned by blargg.
> No one is going to make their emulator 3x slower to properly emulate one glitchy line in Super Mario Bros. that shows up on 1/3 of powerons based on a random clock alignment issue.
If you agree with the premise that an emulator should strive to... emulate the real thing as closely as possible, then it is most certainly a bug.