It's a whole lot less about taking it seriously and much more about timing of the event and general preparedness.
The top league in Cuba is in mid season at the moment and took a break for the WBC. They are reasonably well prepared.
The Asian teams not only started playing in the WBC sooner (March 3), but also were playing exhibition games back in late February against the defending champs (Chiba Lotte). The Japanese pitchers are well into 100-pitch stamina. In round 1, pitchers were supposed to be limited to 65 pitches. Japan's 3 SPs all went 60+ and averaged 64.33. Peavy threw 27 in the US Opener. Only Clemens went 60+ and he was the last SP to make an appearance in round 1 and had the most time to get ready.
The USA Manager is not only committed to playing everyone..but is on the record as stating that he will *start everyone at least once as well. It is spring training for these guys. Of the top 50 players in the WBC with most ABs, only 3 are USA whereas there are 8 for DR, 8 for VZ, 8 for Japan, 7 for Cuba and 6 and 5 for everyone else. If you want to win, play the same 8 guys every day and bat Ichiro leadoff...no problem. The USA team, like it or not, has other priorities.
For a lot of MLB players, on all country teams, it seems to be mostly an issue of timing and preparedness. Makes sense given the offseason rust and the timing of this tournament.