I can edit the above post if you wish, in reference to the word "chargers"
Let me know and I will replace with "adapters".
I agree the certification increases the cost of the adapter.
Which in part I assume is why Tesla worded the policy "automakers" assuming the adapter has been responsibly and thoroughly tested by the auto manufacture to not cause any damage to the it's vehicle charge port/electronics (ie temperature sensing) including the supercharger components as well, part due to the certification process.
If there ever is an issue with an auto manufactured adapter, Tesla could (again I assume) through their contractual agreements deny that manufacture from accessing the network, something that they would have little control of with third party manufactured ones.
If those third party manufactures get the ULC/ certification label, the cost will increase accordingly. It would be up to them to pay Tesla (thru their own contractual agreement) to have access to the network. The cost I could see as being much higher for the certification than one auto manufacture as accredited documentation would have to be provided to demonstrate/prove that their adapters can be used safely with "many" EVs including different voltages and charging rates before Tesla re-writes it's policy.