All airlines do this.

Imagine your reaction if they didn't "reserve" your tickets but instead allowed anyone else to come along - while you are working through your reservation - and grab "your" tickets at the same price while you're still slowly working through the purchase process. If that other customer got through the reservation and payment process faster than you - which wouldn't be difficult given that you stalled your own reservation - you'd then be informed: "Sorry! The price you were quoted is no longer available- The price has now increased by XXXX" or - even worse "Sorry! There are no longer enough seats to fulfil your order. Please go back and reselect your flights"

(You might have noticed this also on a site like Ticketmaster. They give you 15 minutes (usually) to complete your data input and payment, thereby preventing anyone else from swiping those tickets (which would be very likely for in-demand events))