You are right, but....

1. 100% they can get visa from consular next time.
2. 100% they can enter USA on the second entry.
3. Mostly likely they (B2, NOT B1, Especially old man) can get much longer then one month.

Extension may cause your next visa refusal by VO.
1. They had an agreement with VO on the first entry, their invitation letter supposed to show the period they are gonna stay. So come back on time, establish the credit.
2. Technically, they don't have agreement with VO for how long they are going to stay on the second entry.
3. Visa is from consulate, Extension needs permission from INS. Guess they didn't mention the extension to VO when applying for Visa. Even if they get extension from INS, they still face a little possibility of visa refusal next time (especially they extend the visa on the first entry).
4. We all agree that applying for H1, F1... from INS IN USA is easier then OUTSIDE USA. You may get some protections when applying for visa in USA according to immigration law. But you can't benefit from immigration law outside USA.

Consulate or Embassy is DIFFERENT. They are always right to refuse your Visa. Personal advice, don't extend the parents' visa unless you have to do it.