Here's the thing about homeless people in libraries.
Yes, we're ethically and legally obligated to serve all citizens, and we endeavor to do so. But we also do have basic rules of conduct. If someone who smells of stale urine and body odor badly enough to make your eyes water decides to camp out on a bench in your reading area, that disrupts the ability of others to use the facility. If someone decides to bathe in the only restroom and locks themselves inside for forty minutes, the same. If someone responds to an attempt to talk about an issue like that and what can be done to resolve it, available services for the homeless, &c., by waiting until the librarian leaves and then going back into the stacks and urinating all over a shelf and the books on it, that's a problem.
We're there to provide resources and services to everyone, but if someone is actively disrupting the ability of others to use those resources and services, resists attempts to resolve the problem, argues with or threatens staff, &c., it's beyond a matter of "Oh, that poor person being discriminated against." There are plenty of homeless folks I know and see who are perfectly fine and cause no problems whatsoever. There are some who have issues of some sort and habitually cause problems, stalk staff, stalk children, &c. And then there's what happens when the city jail releases a bunch of petty offenders due to overcrowding and they all hop the bus line over because there's a bank to panhandle outside of, a liquor store to hang around, and a public library to misuse.
And that's at a branch a decent way from the city center. A main library is much, much worse to work at--there's a reason why they tend to have permanent security staff rather than maybe a part time rent-a-cop from time to time. It's all the issues of working in a store or restaurant in a city except that people need to be a lot more actively hostile/disruptive/creepy before you can kick them out.
If a public library has shitty furnishings, it's probably because they're badly underfunded and can't afford to replace stuff that's ruined by patrons, much less buy cushy things. Chairs and such with deep, comfortable seats are easier to damage and become much nastier much more quickly. :|