Sounds like you're required to be online during 1PM - 6PM, probably for communication purposes. It's really not that bad at all.
That said, the expectation is not that you're sitting idly at your computer for an extra 15 minutes at the end of the day if you've knocked out your work and are at a good stopping point. We hire adults and treat them as such :)
Also, many folks on the team have familial and other responsibilities that they need to tackle during this window, and the team is very flexible about scheduling around these (especially given advance notice when possible).