Apart from only offering this plan for Malaysia and Australia, you forgot to mention about how Singtel only bundles 1GB for this $10/month dataroam plan. And you can be sure they won't expand this to other countries because it cannibalises on their higher tier plans which cater to many other countries. Just take a look at their price plans and the data it provides. It's really in WTF territory.
M1 on the other hand uses 3GB/4GB of your local data for a flat fee of $12/month and offers this plan for over 17 countries.
In terms of better value, there is no contest between the two and M1's value for money trumps whatever customer service that Singtel provides. And you see this being reflected in each company's entire product offering as well.
I have a feeling why they are unable to offer a similar plan to M1's is because they they were caught off guard by M1's incredibly value for money data passport plans. With the smartphone revolution, Singtel has since not been able to earn as much as before from overseas calls and SMS and have turned to overseas data as their next cash cow. So what you're seeing now is Singtel trying to compete in ways other than price (unlimited data usage on sundays, singtel wifi etc.). It is quite obvious that their management is still reluctant to accept lower revenue growth as compared to their heyday.
Just look at their legacy data roaming plans which cost a whopping $19/day. Although its unlimited, practically speaking, a tourist only needs enough data for the occasional web surfing/google maps/whatsapp/social media.