Of course it is.
We are a sovereign country with an elected govt.
Australia/New Zealand were much more so "obligated" to the Empire at that time.
Australia and New Zealand suffered greatly in treasure - human and otherwise - by allowing Britain to use their man power.
Id need to study more if the use of ANZAC forces at Gallipoli was a case of "expendability" or underestimating an enemy.
When the US entered WWI Pershing ensured its forces were going to be under American command. He saw what had happened under allied command from 1914 to 1917 and I don't think he wanted part of that meat grinder.
There was certainly some pressure from the British Empire on Australia to participate, but they were a sovereign nation at the outbreak of WWI the same as the US. That being said, I'm definitely not an expert on the politics and powers of the British Empire around the turn of the 20th century.
Here are some rough numbers on Australia's participation in WWI (keeping in mind they had been a sovereign nation for about 13 years at this point:
Total population: 4,949,000
Males between 18-44: 1,077,000
Number enlisted*: 417,000 (8.4% of total population, 38.7% of adult males)
Number KIA: 60,000 (1.2% of total population, 5.6% of adult males)
Total KIA, WIA, and POW 216,000 (4.4% of total population, 20.1% of adult males)
*There was no conscription for foreign service in Australia during WWI. All participants were volunteers.
To put that into context, about 10% of adult males in the US enlisted (another ~13% were drafted), 0.6% were KIA, and about 1.6% were KIA, WIA, or POW.
None of this is in any way to diminish the sacrifices of US troops or the impacts on the US populace from WWI, just putting into perspective how much the war rocked an infant nation. Also, these are rough numbers derived from multiple sources, so definitely don't quote me on these, but they illustrate the point nonetheless.