It is not uncommon, more countries practice this and warn before that difference in fees will not be refunded.
A visa should match with what your reason is to visit a country, it is not a kind of entrance ticket you buy for a certain period, and then gives you to do what you want in a country.
That is your opinion, but it is also the responsibility of the applicant to choose the correct visa. When you select wrong visa, they can correct you, or reject the application. I think the correction is much more customer friendly.
saying nothing about retaining means it should be retained the whole period.
If it was allowed to let it drop, it would be described as with non-o retirement.
They will only check if they for some reason suspect you, or just coincidental because they might need to do a certain number of checks in a certain time.
When you comply to the visa requirements the in country extension might be the best way to go. When you do not comply anymore doing a bordebounce yourself might have the risk questioned and checked for funds and documents, unless you pay.
The country tries to get people in who actually spend money, for that purpose they have attractive visa.
I have no problems with backpackers, but the economy does not really profit from them, and some of them also work to pay for their living, and that is something Thailand wants to prevent. Backpackers only spending a few months don't get problems with visa exempt, but living in Thailand just is a different thing.