Polices tend to be lab and department specific. Most everyone I know if reimbursed for flights and hotels (though they are typically required to apply for internal travel grants that exist for this purpose). Food reimbursement varies from lab to lab.

In my experience, in the US, it is normal to reimburse physics graduate students. I won't say how many of us there were to a room at times, but we were reimbursed....

CS PhD here: In our field, yes.

In my department, typically each student has a finite amount allocated for conference travel. After that, you either apply for travel grants, or (commonly) your supervisor will probably have access to extra funding, but it's still a finite resource.

Nobody would go to a conference if they had to pay their own way because nobody's paid enough to be able to, quite frankly.