There is NO reason why someone can be notified after they return from PTO. The decision does not change as a result. Just using some basic sensitivity is not hard - if you want to be sensitive.
The decision to tell people while on PTO or after they get back is a hard one and is a lose/lose situation. Some people don't want to know because it ruins their vacation. Others want to know so they can cut out any spending they don't need to spend, or possibly cut the vacation short if they can to cut costs. Even better would be to tell the employee BEFORE their vacation and make them sign a non-disclosure agreement (NDA) so they can either take or cancel their vacation knowing that they're going to suddenly be unemployed.
I had a company lay me off the Monday following Thanksgiving. I had taken Friday off to make it a 4-day weekend, so it didn't really bother me, but that company based it's 401(k) matching for current employees at year-end and made a lump-sum matching contribution the first week of Jan for all employees a/o Dec 31st. They notified me that they were laying me off on 11/28. I got 2 weeks severance in lieu of notice & an additional 2 weeks pay if I signed the agreement not to sue. If I signed, my employment ended Dec 28th and if I didn't, it ended Dec 14th. So I didn't get any of the 401(k) matching for that entire year's contributions. That is what I thought was really cold hearted of them.
To top it off, the office admin who was in the conf room with me while HR was conferenced in on the phone tried to tell me it was a "generous package" as well as providing me with a copy of the agreement I'd signed when I was hired agreeing that when I left the company I would not go to work for any of their customers, suppliers or competitors for a period of 3 years.
I thought it was hilarious when, 95 days later, the company closed the entire office and laid off everyone except the handful of senior execs. Turns out they had let just enough of us go at Thanksgiving (technically Dec 28th) so that they could let 48 people go on March 1st and avoid the WARN act's requirements of providing job placement services. Damn bean counters. I had to ask the office admin if she thought the package was still generous when she was the one receiving it and how she felt about the non-compete requirement since it limited her employment options.
I didn't give a c-ap about the non-compete. I interviewed with any company with an opening matching my skills and would let them decide if it was worth it to take me to court over it. Turns out it wasn't.