The question, I think, that you are asking is: can a company legally accept money from public purchasers in exchange for its hashrate?

The answer is: It depends.

For United States issuers, at least, I can add some detail.  The way that most companies have been going about this is by selling "perpetual mining bonds".  These things - whatever else they might be - are securities subject to the registration requirements of the Securities Act.  That means that the company must pay a princely sum to attorneys and often auditors for securities compliance work. For small issuances like the "IPOs" we've seen, that just isn't cost-effective.  Furthermore, they can only be sold to a limited number of non-accredited or unsophisticated investors.  Issuers have essentially ignored these requirements and sold them anyway, taking investors money in exchange for little more than a promise to pay them back out of whatever profits they generate.  That's not legal.

I think there is another way, a better way, to do this, and without violating the securities requirements.  Instead of selling a right to profits or equity in the company, a company might sell a right to the hashes themselves not tied to the performance of the company.  Just like Amazon might sell its terabytes on a Glacier server, a mining company might sell a subscription to its hashing power directly to its customers.  Key word being customers - not investors.  You'd need a lawyer to prepare a proper contract for you, and it wouldn't be cheap, but I think it can be done.  Also, I think a competent attorney could prepare a contract that might even be freely assignable to other customers.  I'd love to hear the opinion of any legal beagles out there.