This is an interesting story and raises an interesting question:

From DNAinfo.com:

An Australian couple known for their hospitality is butting up against a Central Park West condo board full of hostility.
Gavin and Jodi Samuels, who rent out two apartments on the fourth floor of their doorman highrise, regularly host large Shabbat dinners, special cooking classes and rent their place out to guests for over 30 days at a time without board approval, according to a new law suit filed in Manhattan Supreme Court.
Now the Olmstead Condominium board wants a judge to put the married couple's guerilla bed and breakfast out of business, claiming that it violates the building bylaws, disrupts the other tenants and taxes the building's elevators.
Jodi, a former business advisor for Australia National Bank who now runs a non-for-profit organization catering to ex-pat Jews, seems proud to share her home. On Metroimma, a website for Jewish mothers, she boasts that she has so many guest she calls her place "Camp Samuels."
"We have the travelers, the new arrivals, the sacred, the lost and the lonely," she wrote. "There is always someone in our home and often more than once."
And the professional couple seem to be making out nicely from their hospitality.
"As usual, we rent out our New York apartment at a premium...it actually pays for us to go away," Jodi wrote on the website.
But the foot traffic in the hallways appears to be bothering the other tenants.
Lawyers for the board point out in the lawsuit that city looks down on internet apartment subletting like Airbnb.com and Homeaway.com that has recently become popular.
In January, condo boardmembers with time on their hands dug through those sites and found the couple had rented out their space five times in 2012, usually for less than two weeks.
The board claimed that the couple also charges $25 to $30 per person for Shabbat dinners, which they claim draw about 40 people a meal or about 1000 guests a year. Neighbors not only have to put up with the guests, there is and increased burden on the staff and elevators, they said.
A regular Shabbat cooking class that is held in their home brings in $15 per person, the board claimed in the suit.
Even after they asked them to stop with the bed and breakfast the couple continued, earning them a $5,500 fine from the board.
The suit has asked a state judge to force the Samuelses to end the party and make the couple fork over $25,000 for their violations.
The couple could not be reached for comment.

If they are actually breaking conditions of their contract, there would probably be no question. If the contract says no subletting, they should not be allowed to sublet, and doing so would be a violation. It should not even be much of a dispute. If the by-laws talk about not bringing in x amount of foot traffic, not selling meals, not bringing in over a certain number of guests, that might be an issue.

the way it is described, it sounds like neighbors are angry, perhaps justifiably so, but if no conditions are being broken, I don't see what they are doing wrong. It says their behavior is against the by-laws, but does not reference what the by-laws are that are being violated. It sounds like that is a bluff and they just want these people to stop. Or else the article is simply lacking important information.

I wonder if this couple had guests for free rather than for money would the neighbors still care as much.

They definitely should be more considerate of their neighbors, and perhaps they should offer or be made to pay a higher building maintenance fee due to their higher usage of floor traffic and elevators, but I do not know if they are actually doing anything wrong..