I believe this originates from GNU argument syntax and is commonly implemented by software supporting GNU-style long options. Terminating after the first argument IIRC is what software implementing BSD style argument parsing usually does. OTOH though, I am sure there is software that implements hybrids of both approaches. I don't think any approach is particularly "weird" really, both of them make perfect sense in isolation.

edit: relevant GNU docs https://www.gnu.org/software/libc/manual/html_node/Argument-...