Sorry to contradict you Gutenberg, but it depends entirely on the referee. Some consider it more hassle, others are happy to just print them off and give them to you. Of my two referees for PG applications, one was happy to print off the reference and let me look and upload it myself, whereas the other insisted on sealed and stamped envelopes. The latter refused to budge when I told her I required an open reference to upload. Both are senior figures and group leaders, as such they have provided references before and understand the process.
I applied to a few places, one wanted references uploaded electronically (a scanned copy), one was a paper application (no electronic process available) and another contacted the referees after the application was submitted to obtain references directly. This kind of situation isn't uncommon, as there is no centralised PG applications system, so some academics may consider it easier to provide the copies of the references, either sealed or open as required, and you attach them to the applications and send them on.
In case you're curious, regarding the referee who only provided a sealed reference and the online application requiring a scanned copy, there is usually a postal method for providing references which we went through. Be warned though, this is a slightly longer process as the reference not only needs to arrive at admissions but must then be linked to your application before it arrives for consideration.
Regarding sealed references, it's less to do with them not wanting you to see the reference and more to do with ensuring there is no question that it has not been altered. I had a spare sealed reference left after all applications were sent off and I opened it, it was glowing and a pretty good reference. So I wouldn't worry that if your referee refuses to provide open references that it's a bad one