We have bought handicapped seats a number of times. My husband at his best is on a cane, at his worst in a motorized wheel chair. At any time we leave the seats vacant, the general public sits in the seats because they are convenient to the aisle and the mezzanine, if you don't have an aggressive usher keeping them out. This is during the actual event.
To give the public their due, the seats are sometimes not explicitly labelled as handicapped. Maybe they think those fold up chairs are for the ushers? When Mr. Ski is in a motorized wheelchair, his seat at least is secure. It's mine that has some snotnose sitting in it when I come back from the concession stand with a tray. Evicting them is never pleasant. When it's an entire group that has moved in, like Michael Weiss's family, whom I've seen do this twice (Nationals in Nashville, TN most egregiously). The handicapped people and/or their husbands or wives generally just try to survive until Michael had finished skating, and the family moved on again.
In an open practice, the only defense of such seats is that they are not right next to the ice, and are not the preferred seats if you don't have to use them. However, people take them as casual seating on the way to the concessions quite frequently. It's one reason we don't go to open practices deliberately. However we do arrive early for the event, which may be preceded by an open practice, because there are apt to be difficulties setting up-and sometimes there are people in our seats. Sometimes, as described above, it gets nasty.