Hmm I should have thought my question out more throughly before I wrote. I don't think they're doing anything illegal. This is the first time I'm doing anything like this. I'm wondering how one is supposed to look into other opportunities if everywhere they apply will just withdraw the application. Do they expect people to quit in hopes that they'll get the employment? Or should I not have said anything? Unsure of what to do in this situation. I just don't want to be put into a job that I don't enjoy because money is being thrown at me, so I started to seek other opportunities.
It's totally acceptable to apply to multiple companies and negotiate with multiple offers in hand; you could say to B, "Well, A is offering me ${FOO} a year and two week's vacation, but I'm really excited about the work at B; can you match their offer?" or something like that. But once you accept an offer, it's generally understood that you stop looking for a little while.
It's also totally acceptable to look for a new job while you're working; people get jobs they don't like, or jobs change, and people want something new.
But what it sounds like you've done here is applied at a company, gotten an offer, accepted it, and started looking for new offers immediately. That's unusual, and is definitely setting off some alarms for HR. You're worried that you won't like the job, but there's no way for you to know; you haven't started yet. You're jumping ship before you've even gotten on board; why would B extend an offer to you? How do they know that you won't accept an offer with them and then immediately start talking to C?
The thing to do is to not accept an offer you're not happy with; if one company gets back to you and you're still waiting to hear back from somebody else, it's okay to say "Can I have some time to think about your offer? I'm still waiting to hear back from some other companies, and I want to make sure I have a good understanding of my options before committing". And then you can say to the other companies, "I need a decision from you soon; I have an offer from A and I want to respond in a timely manner to them."
edit: Also what patio11 said; he is eminently qualified to give job-search advice, especially with regards to things like negotiations.
I'll admit it I regret accepting the offer so willingly. It was an exploding offer and they gave me about a week to accept. This was one whole year before I even graduated. I'm sure this has allowed them to lock me in at a lower salary than what they are offering new hires now as well. The whole situation just makes me feel a bit bitter about the job. It was definitely a learning experience for me and I wont make that mistake again. Just trying to figure out what to do now. I don't want to establish a career in a field I'm not interested and lose the momentum I've already gained in my field of interest.
