You don't want the employer to waste your time, so you should try not to waste theirs. If you want to work for them you should go in assuming they're not trying to deceive you either.
You counter offers for a dozen other things, why not time? All those other negotiables can just as easily blow the "deal" apart but aren't considered a waste of time/deceit. They often aren't pre-aligned on. Even if you pre-aligned on something, the interview process itself can change your opinion. For example, you may agree that a salary of $x would suffice. During interviews, you determine the job is going to be much more demanding than originally expected so you now say you like it but $y is the new price. It's not deceitful, it's quite normal.
I agree your example of a salary adjustment is fine, no deceit there.
The deceit comes from pretending to be qualified for a fundamental job requirement that you'll never meet (the requirement to work full time).
A better example is applying for a job that requires a license and after weeks of interviews you finally get an offer just to turn around and say "actually I don't have a license and I'm not willing to get one, but I hope your offer still stands?". Except it's even worse becuase at least in that case the employer can verify your license right off and avoid wasting everyone's time.
Lying on a resume or application is just not a good look.
You don't want the employer to waste your time, so you should try not to waste theirs. If you want to work for them you should go in assuming they're not trying to deceive you either.