No, it's not rational to demand more from someone who you pay more. Negotiations are two-sided.
If, for your $5,000 pay bump†, the company decides they'd like you to be closing 35% more bugs than every other employer in your pay band, they can demand that in the negotiation. You can then make a decision about whether or not the bump is worth the concession. If it isn't, you can decline the job, or you can negotiate the concession, or you can decide to accept the previous offer.
It is (word chosen carefully:) alarming how bad engineers are at negotiation. Beliefs like yours (aggressive negotiation entitles an employer to maltreat employees later on down the line) are alarmingly common. They're simply not true.
The detail-shackled brains of nerds†† seem unable to get past something here: no matter what employers say the feel about negotiations, surely they can register the tenor of the negotiation in the back of their heads, and then ratchet their expectations appropriately. Surely, nerd engineers think, that must occasionally happen. Answer: yes, it does happen. AT BAD EMPLOYERS. Good employers understand how salary negotiations work and allow for it. The best employers even account for how bad engineers are at negotiations and go out of their way to help educate candidates (Fog Creek, for instance, once said they often "top up" the best offers some candidates receive --- that should function as a big red flashing light to those people that they didn't negotiate well enough).
I am not telling you that employers won't do irrational things if you negotiate hard with them. I'm telling you that if they do, you should run run run away fast, because that's the canary in the coalmine. Those employers are also going to dilute the shit out of you, call you in to work on weekdays, swap your tolerable manager out with some douchebag they hire from a BigCo to increase productivity, lock you into a regimented COLA-based incentive comp plan, slash your health care benefits 8 months down the road, and never ever improve the hardware you're issued.
Don't believe me? Think of it this way: the best engineers are disproportionately likely to negotiate aggressively. Employers that get vindictive about negotiation don't get to hire those people. Why are you interested in working at an employer that shuns the best talent?
(Hiring manager, one of many at my company, speaking here.)
There. You got a prolix response from me. Happy? :)
† (about which Patrick is right: its a rounding error, and it's the kind of rounding error that --- unlike, say, a better- but- out- of- spec laptop, which might incur a huge companywide "new laptops for everyone" shitstorm --- is particularly easy to concede to a new hire.)
†† (see my about:)