I don't personally have a dog in this race because I'm happy working a single job, but I think you might be making assumptions that don't necessarily apply to all full-time jobs.
If at your company, "duration of time someone's butt is in their seat" is a meaningful business measurement, then by all means, insist your employees spend the required amount of time in their chair. Most full-time jobs I've had did not specify hours with such rigidity, and the best ones were very flexible about other things we need to do during the day (even, gasp, during business hours). Your business might not be willing to provide such flexibility. Definitely ask candidates about side projects and other jobs during the interview process and make your policy clear so they know it's important. Disclosure goes both ways.
I would find it distasteful if I got fired (and then the firer reached out to another, unrelated party to continue the damage), over something that was glossed over (or never asked about) during the interview, and simply written somewhere on page 13 of the employment agreement.
Just to add to it, looking from time perspective you feel like a fool, there were many occasions in span of several months when SU/meeting couldn't be attended; already constrainted due to timezone difference, you'd think he'd make an effort to at least be available during short overlap hours, but couldn't because:
- had to pick up mom from airport
- had doctor's appointment
- electricity was cut off the building
- mom is out of medicines
- traveling back to my hometown
- still at airport going back home
- checkup with my mom
- at hospital
- my mom is getting gallstones removed (she was fine after operation!)
- have dental appointment
- i have follow up checkup
- have problems peeing for last days, decided to check at it at hospital
- wasn't able to check in yesterday sorry
- not feeling good today
- can't attend
- driving back from pharmacy
- had some headache this morning
- can't join accompanying my mom for her monthly checkup
- can't attend bought some meds for mom and got stuck in traffic going home
- power interruption in my area
- picking up my sister at the airport
- had fever
- still not feeling well hopefully my tonsils will heal tomorrow
- internet today is on/off in my area maybe due to weather
- my internet is off restarting my router
- i'm on m1
- dropping my brother at airport
- having connection issues
- there was line cut off near our area
- having issues due to upgrade in our area
- currently travelling to my province
- heavy traffic on friday
- can't join will be driving my brother to the airport and there can be traffic
I had this happen too. I started a spreadsheet to keep track just to show my bosses that I wasn't crazy, something seemed very off. The excuses I got were more questionable than these. I got very lucky when they were let go as part of a layoff, never had to really confront them about it.
You can say you'd like to perform work for two different companies and negotiate your working hours and conditions with both.