IIRC, careers beyond the first get one skill at zero-level from the list of service skills of the new profession. But I had the same reaction as you when I first read the MgT Ruleset.
I guess it is to keep CharGen from granting too many skills to people who constantly switch careers. But it still seems somewhat illogical.
I don't have access to the book right now, but didn't MgT have a mechanism that limited the total number of skill levels that a character could have? That's one brack on powergaming for excessively skilled characters.
The other is that if a character changed professions a few times, while they may get a number of skills at level 0, their total number of advances beyond that would be limited, wouldn't it?