The reason some churches use electric votive candles is to reduce the possibility of fire hazard. It is my understanding that in certain places this is even required according to local fire codes. Votive candles are designed to burn for a full day, or even a full week, so they will be burning at times when the church is unattended (and therefore no one is around to stop an accidental blaze before it spreads).
This is not an issue for altar candles because they are only used during certain liturgies, and are extinguished when the liturgies conclude.
My parish is able to use actual wax votive candles, and there is a donation box next to each set. There is no sign designating a specified donation, and people may freely light candles even if they have no money to donate. So far, the amount that gets donated is sufficient to handle the cost of the candles, and if not then the pastor is willing to make up the difference from the parish's offertory.
But, of course, there are probably lots of places that at one time used my parish's system but not enough was collected to cover the cost of the candles (or the electrical expense in the case of elective votive candles) and therefore a specific donation was imposed. If that is the case then I really can't fault the parish for establishing a specific donation rather than allow the situation to be a drain on the offertory.
Nevertheless, it makes me cringe somewhat to see a statement about a required donation because the terms "required" and "donation" are contradictory. I think the term "suggested donation" is better, with the understanding that it is, indeed, a suggestion and not an obligation. Once someone is told that he is obligated to make a specific donation, then it is not truly a donation, but rather a fee or a price.