Would it be correct to say that their maximum allowable match would be 6%?
That's my take on it too. But likely am wrong.
If so, why is it advantageous for them to structure it this way?
It honestly doesn't make sense the way they have it structured, but I am not an expert.
Matches based on the employee's contribution level are mostly an incentive for the employee to save more, and a benefit in recruiting new people. It is a great benefit to attract new hires and acts as a supplementary form of compensation. And if there is a vesting schedule (and there usually is) it also acts as a retention incentive so you have less employee turnover.
The way they have theirs structured, it likely is to take advantage of all the previous, but also because so many folks rarely even enroll in their 401k, let alone fund it past 3%. So it's likely a great recruiting bonus and a savings (for the company) if most don't go over 3% contributions. They can still say they offer matching, but most folks never contribute enough to earn that full percentage.