I recently had a discussion with an ebay_maker of a board ($30) on some really annoying issue - the answer was "we always try to balance quality and price, thanks for hint, we may improve in the next batch"
Most probably a "designer" got the task to reduce the number of pins on the header from 4 to 3.
So he/she wired the optocoupler's diode to only voltage available: +12V. Task done !
The dangers of having connected ground and 12V Vcc of that board to a low voltage MCU board are mainly 2:
1. the relay coil, when not protected by the parallel diode (it can break as well), can produce hundreds of volts peaks and destroy the transistor and propagate via 12V rail
2. the relay contacts, which may drive 230V/10A can break and short to the coil and therefore to 12V rail.
PS: another small detail - always ground the base of a transistor (bipolar or fet) which switches loads via a Base-Emitter resistor (see my schema for example) - when left base floating (when connected directly to an MCU's gpio and the gpio is set input or analog, ie. during reset) or at higher impedance (ie when the optocoupler's transistor is off) it may randomly switch the collector load off/on/off, based on the noisiness of the environment.