I think its to ensure that Tesla doesn't just use a loophole to get into the program. Tesla will be losing money on the crippled SR- and the government doesn't want to let them recoup that lost revenue by allowing a software upgrade....basically they want to have a 'real' EV at that price point, not a virtual one.
In the extreme, as I mentioned above, Tesla could make a special Canadian Model S at 44,999 that is horribly crippled to 10 km range and a top speed of 50 km/h, and immediately offer an upgrade for whatever the cost delta is to a real Model S. Technically, that would qualify for the rebate program...but that would be a little shady. Admittedly an extreme example, but in my opinion its going to be viewed the same by the general public....and it always comes down to political optics....
Also, there is this statement at the end of the list of eligible vehicles:
View attachment 403406
So, if Tesla 'did' offer an upgrade from SR- to SR (not SR+) technically the government could remove them from the list, perhaps even negating rebates for cars en route to customers. If however Tesla is up front about 'no upgrade path', then they are complying with the rules. They could likely offer an upgrade to SR+, but it seems risky to offer SR- to SR.
SR- isn't even a 'thing'....its really the Canadian SR version we are talking about since you cant order regular SR here anymore, but you can in the US. I'm just too lazy to write 'Canadian SR' every time