It's not clear what sort of answer you are looking for, but aspects of this cannot be answered globally, because there is no global "law" overseeing how RF equipment is sold.
First, a bit of a challenge: in most places in the world it is legal to create your own transmitter for amateur use only if you achieve some level of advanced qualifications. For a majority of licensed hams, we are specifically not allowed to design and build a homebrew transmitter. Perhaps there is an HT kit, in which case that would be legal to build.
There are only a few major regional markets for this equipment, and a handful of transnational corporations that make the equipment. Over the years these regions have introduced tighter requirements which has created multiple market streams of equipment. For example, the FCC and Industry Canada have harmonized their requirements for RF equipment such that it is nearly impossible to buy a device that is "unlocked", because all the major brands have complied with those requirements in order to get the highest level of certification for those regional markets. I'm sure the Euro zone has a similar situation.
International trade is tricky to police, however, and there is still opportunity for grey markets for manufacturers that don't care about actual certification. This only lasts so long as the FCC, et al, slowly close those loopholes over time. Many of the Chinese manufacturers that used to offer equipment that worked everywhere, and depended on the user to make the right choices now sell through regional resellers specifically so they can lock transmission to specific bands. Eventually all Chinese manufacturers will end up doing this, or risk being lumped into grey or contraband markets. A Baofeng HT can only generate so much profit at such thin margins if containers arriving in ports keep getting flagged and sent back to China, or dumped on the grey market at a discount.
This has created situations in adjacent regional markets where you can't seem to buy equipment that matches your actual privileges. For example, both Canada and the UK can't do much about the US and Euro agreements, respectfully, that unnecessarily restrict some band access. Some manufacturers offer to address this for you, for a price. But there isn't much you can do about being a minor market overshadowed by a much, much larger one.
So, this is what is happening. The question here is "why" is it happening. That can't be easily answered here. Someone needs to ask the FCC, or the Euro zone bodies in charge why there has been a shift from the onus of compliance being on the manufacturer rather than the user. But we can guess that the people operating the equipment over the years may have not demonstrated the maturity or knowledge required to use them in a compliant manner.
In short, it looks like these spectrum management policies are about making the defaults as idiot-proof as they can, knowing nature will just build a better idiot. Paying extra to unlock the equipment may indemnify the manufacturers in some manner while offering a sort of gate. If you are clever enough to know you want the transmitter unlocked, then that is a very clear indication that you are willing to also accept full responsibility for doing so. Whereas, someone just picking up their first HT while they work on their qualifications may not even be in a position to know what they are agreeing to. Thus, you create a default device for the region for that large market, with an exception for those who say they understand the risks.
