Does Google+ come preinstalled on Android phones? Is having it a mandatory requirement on manufacturers to be certified by Google?
Google Plus is not shipped typically as an standalone app but a part of the bundle named Google Apps (GAPPS).
The core of the GAPPS package is the famous app named Google Play Services which is accessible directly by the user in the form of Google Settings.
That said, an OEM isn't allowed to ship Google Plus or any part of GAPPS package, without the consent from Google. Google on the Android Compatibility page has stated that:
Licensing Google Mobile Services (GMS)
After building an [Android compatible device], consider licensing Google Mobile Services (GMS), Google’s proprietary suite of apps (Google Play, YouTube, Google Maps, Gmail, and more ) that run on top of Android. GMS is not part of the Android Open Source Project and is available only through a license with Google.
To get their hands on GMS apps, OEMs need to license them from Google; and in order for a device to get a license for the apps, it must pass the Android Compatibility Test Suite and meet the Android Compatibility Definition.
Is having it a mandatory requirement on manufacturers to be certified by Google?
I can't answer this part satisfactorily since I'm restricted to my opinion. However you may take a note on the info in the aforementioned link. It has mentioned that certain OEMs are contractually bound to not to produce non-Android compatible devices so as to avoid Android fragmentation which would be inimical to Google's business. The contract itself isn't released for the public view, so you're possibly on your own.
As for the part why the users are being forced to use or at the least have them in their device, you should consider asking your OEM first! :)
Edit by Izzy:
as Firelord asked me to include it, I'll append it here:
There's an article on Engadget on the Google Mobile Application Distribution Agreement (MADA), which describes some of the background. Simplified, if a manufacturer/OEM wants to include Google Play Store (which most want), they must not only include a bunch of additional Google apps, but also place them on the main homescreen page. So yes, this is "a mandatory requirement on manufacturers to be certified by Google" – not only for Google+.